I have recently joined a 5e campaign and I was thinking about making a Sorcerer Rogue multi-classed character. Before I commit to that character build, what are the pros and cons of multi-classing in the 5e stystem that apply to mixing those two classes: Sorcerer and Rogue.
Thou shalt not sacrifice caster levels.
As the golden rule of 3.5 multiclssing, it's hard to beat the above. It's quite possible to apply requirements gathering methods to 3.5 as well as 4e, and estimate approximate to-hit and damage at each level.
Plan out your character to 20.
s there an effective way to identify class combinations which seem appealing up front, but which will fall behind as the characters level up (without actually taking the character through its paces in a game)?
is simple: plan out your character to level 20. Identify, at each level, what benefit the character is deriving from your choices. Test against the requirements given in your requirements step. This way, when your character shows up in game, you have an idea of your intent and the capabilities of the character.
To answer what classes synergize well:
Non primary-casting classes tend to synergize well.
Given that most aspects of a level up are cumulative with prior choices (BAB, feats, HP) you want to avoid class features which depend on your level in the class. Therefore, avoid class features which have level as a variable within the feature.
Rages from barbarian are fine. You get more as you level up:
a barbarian can rage for a number of rounds per day equal to 4 + her Constitution modifier. At each level after 1st, she can rage for 2 additional rounds.
but you don't have the benefits of the rage reduced (proportionally to your compatriots) as you see in spells.
For the casting classes it's certainly possible to combine them in "gishy" characters (spell-slinging fighters) but they require a specific focus into the combination, preferably enabled by appropriate choice of prestige classes.
So, if you fail to raise rogue, you don't get as much sneak attack bonus damage, but if you're increasing fighter as the other class, you get a higher BAB to compensate.
Whereas a rogue would effectively be writing off "improved uncanny dodge" due to multiclassing.
Therefore, a good combination can be fighter/rogue, because each class brings a separate thing to the table, without having a significant opportunity cost for choosing the other. Other good combinations are those that lead up to exceptional PrCs, which combine features from both classes in useful synergy.
“There is no point in doing this [from a game mechanic perspective].” The sorcerer is just the stronger class, and even the feature-heavy first level of bard does not compare to simply having better spells sooner (see 1. Spellcasters should not multiclass in this answer for more details). Thus, the best mix of nine levels for bard or sorcerer is Sorcerer 9. That said...
The best way to multiclass bard and “sorcerer” is to not have any sorcerer levels at all, but rather take the sublime chord prestige class from Complete Arcane. This class requires Bardic Mustic and 3rd-level spells, but rather than progress bard spellcasting, it has its own spellcasting with 3rd- to 9th-level spells from the bard and sorcerer/wizard lists. This spellcasting is Charisma-based and spontaneous. It also progresses bardic music, and gives several special “magic themed” songs.
So a Bard 10/Sublime Chord 10 casts as a 10th-level bard and also has separate spellcasting with spells of up to 9th-level that come from the sorcerer/wizard spell list (or bard list). It has the ability with music of a 20th-level bard, except some of the songs are changed to be more “magic themed.”
Since spellcasting is the only sorcerer class feature aside from the familiar, having spontaneous Charisma-based spellcasting off of the sorcerer/wizard list, and then taking the Obtain Familiar feat, makes you effectively identical to a sorcerer. But this progression is much smoother, you end up with level-appropriate power at higher levels, and sublime chord is really cool. The only problem here is that, before 11th level when you take your first level of sublime chord, you have no mechanical representation of being a sorcerer. But bard and sorcerer spellcasting are fairly similar, and you can take Obtain Familiar at Bard 1, so it should be easy to continue to call yourself a sorcerer at lower levels.
Worth mentioning: Champions of Valor has a variant paladin, the harmonizing knight, that gets Inspire Courage +1, 1/day instead of at-will detect evil at 1st level. In the Forgotten Realms, this requires you to worship Milil, a goddess of music; in other settings, it would have to be adapted to some appropriate patron. Anyway, all paladins get the excellent Divine Grace at 2nd level, adding Charisma bonus to all saving throws. As such, Paladin 2/Bard 8/Sublime Chord 10 becomes an excellent variant on the above build: you trade 1 daily use of Inspire Courage for full martial weapon proficiency, a bit more HP, +1 BAB, and adding your Charisma bonus to all saving throws. Since your Charisma should be high, that is a very nice bonus. Adding paladin is not an option for all characters, of course, but if it is, do consider it.
For spellswording as a bard, whether you dip paladin or not, I strongly recommend the Snowflake Wardance feat from Frostburn, if you have that book. Other excellent options include the harmonizing weapon property and crystal echoblade weapon from Magic Item Compendium.
Finally, if you have Tome of Battle, taking a level of crusader for the Song of the White Raven feat is an awesome option, dramatically amping up your physical prowess while allowing you to start performing Inspire Courage as a swift action. It also opens up the interesting possibility of using the jade phoenix mage prestige class to advance sublime chord spellcasting, which would be ideal. Note that Paladin 2/Bard 7/Crusader 1 still just qualifies for sublime chord, too, if you want to do both. I recommend taking the crusader level at precisely 9th level, so you can simultaneously take Song of the White Raven, and have Initiator Level 5 so 3rd-level maneuvers and stances are available to you.
Generally speaking, race isn’t all that important; human is probably your best bet just because bonus feats are awesome. But anything without a penalty to Charisma or Constitution is probably fine (bonuses to Charisma are really rare and always paired with a penalty to Constitution, so there is little to be gained there). Even penalties to Charisma or Constitution are bearable, but why would you?
However, in the case of anyone with bardic music, the benefits of being a dragonblooded race have to be mentioned. And since there’s a dragonblooded human race, silverbrow humans from Dragon Magic are almost-certainly your best option. They trade the humans’ bonus skill point for the Dragonblood subtype, which among other things qualifies you for the excellent Dragonfire Inspiration feat from the same book. Highly recommended.