With an ultimate magus, it’s more important to keep yourself as close as possible to a single-classed spellcaster on one side than it is to keep the two even.
Thus you want to circumvent as much as possible ultimate magus’s attempts to progress your lower-level class.
Because at 1st, 4th, and 7th level, ultimate magus advances the class with lower caster level (your choice in the case of a tie), this has a tendency to even out your two classes. If you avoid that, and continue to progress in the more advanced class, you will have more of your highest-level spell slots, which are also higher level than they would be.
Practiced Spellcaster is the solution to taking manual control over which class ultimate magus progresses. Practiced Spellcaster increases a class’s caster level (but no other facet of its spellcasting) by up to 4 (capped at the spellcaster’s HD, so the feat is only beneficial to multiclassed spellcasters). Normally, caster level is nice but not as crucial as things like spell slots or higher spell levels. Ultimate magus switches this up, though.
If a Wizard 4/Sorcerer 1 takes Practiced Spellcaster for sorcerer, his weaker class, his sorcerer caster level is 5th. This puts it higher than his wizard spellcasting’s 4th caster level, so first level of ultimate magus advances wizard. This is despite the fact that the ultimate magus’s wizard spellcasting is, for most purposes, more advanced than his sorcerer spellcasting.
Now you have some control over your advancement, and that allows you to maximize your spell slots.
At 4th level, both classes are even, so you should choose wizard. At 7th, barring finding some other bonus to sorcerer caster level, though, you have to accept some sorcerer progression, which is a shame but not the end of the world. You’ll end up “missing” a total of two wizard spellcasting levels (compared to a single-classed wizard), but gain a total of nine sorcerer levels, plus of course all the rest of the ultimate magus class features.
Otherwise, you need to improve the abilities that grant you bonus spells. Consider the beguiler (Player's Handbook II) instead of sorcerer: you'll use Intelligence for both classes.
Versatile Spellcaster is another feat that will make your slots more flexible.
And pearls of power and rings of wizardry can help by letting you reuse slots. Memento magicka are like pearls of power for spontaneous spellcasters but they are 50% more expensive, which is badly overpriced.
As with most Sorcerers, your spell selection is going to have a far greater effect on your character's playstyle than feats ever will. Once your Luck feat requirements have been satisfied, consider filling the rest up with general, Sorcerer-useful feats, like Versatile Spellcaster (as Swooper suggested), Arcane Thesis (Player's Handbook 2) coupled with Metamagic feats, reserve feats (Complete Mage) and so on.
Unluck (Spell Compendium) and Alter Fortune (Player's Handbook 2) come to mind as luck-themed spells.
As for luck feats:
- Lucky Start is definitely the first Luck feat I'd take. Initiative is one of the few rolls that consistently matters for a Sorcerer.
- Magical Fortune is OK, I guess.
- Tempting Fate is kind of planning to fail, but it's likely to save your butt sometime. So sure, why not.
- Lucky Catch is far too situational to be worth it.
- I'd consider Unbelievable Luck or Survivor's Luck next.
Domain Access: Luck is a complete trap. Notice how many spells known it costs you. You're losing one spell per (presumably spell) level, and only get the meager benefit of being able to cast one domain spell per spell level per day in return as far as spells are concerned.
Regarding your question about Draconic Heritage: If you're talking about the feat presented in Races of the Dragon and Complete Arcane, it doesn't add any spells known to your list. It's the Draconic Legacy feat that adds them.
Best Answer
Your Familiar only improves with Sorcerer levels, so a Sorcerer 10 has a stronger Familiar than a Sorcerer 5/Prestige Class 5 (unless, of course, the Prestige Class explicitly advances your Familiar, which a few do – Alienist from Complete Arcane and Fleshwarper from Lords of Madness, for examples).
But other than that, no. Sorcerers generally have nothing to lose and everything to gain from going into a full-casting Prestige Class.
Of course, be careful about prerequisites; a number of them can be quite painful to Sorcerers (who lack bonus feats and cannot trivially pick up the ability to cast a given spell the way a Wizard can). A classic example of this is the Loremaster: in theory, the class is 10 levels of free class features for the Sorcerer, but in reality you have to burn several feats, somehow get 8 ranks in cross-class skills, and find seven different Divination spells worth spending your extremely-few Spells Known on. (The Wizard, on the other hand, can get in while barely trying, thanks to his class skills, Bonus Feats and arbitrary number of Spells Known.)