For a PFS play, I am planning to prestige my conjurer-specialist Wizard (currently level 5) into the Evangelist of Nethys.
So, level 5 Wizard is
BAB +2, Base Saves +1, +1, +4
But on level 6 the chassis actually get a long-awaited bump to
BAB +3, Base Saves +2, +2, +5
At level 7 these look:
Wizard 5/Evangelist 2
BAB +3, Base Saves +1, +2, +4
Wizard 6/Evangelist 1
BAB +3, Base Saves +2, +3, +5
At level 12, each option will have levels of Evangelist on top of it, making them respectively:
Wizard 5/Evangelist 7
BAB +7, Base Saves +3, +5, +6
Wizard 6/Evangelist 6
BAB +7, Base Saves +4, +5, +7
Looks similar, but for the Seeker tier and further on:
Wizard 5/Evangelist 8
BAB +8, Base Saves +3, +5, +6
Wizard 6/Evangelist 7
BAB +8, Base Saves +4, +6, +7
And essentially Wiz5+E is just catching up in terms of BAB and saves to Wiz6+E all the way, which is most noticable during first levels of Evangelist.
From an optimization standpoint, which will be a better option: going Evangelist for level 6, or delaying one level to get Wizard's BAB and saves increases? Is BAB+Saves comparable to Evangelist's other features: skill points and class skills, divine boons one level earlier, and so on?
Or is it neglectable to the point of the answer being: "whatever, one BAB earlier or later doesn't make a difference, go with what gives you more fun" (which I probably am going to do).
The three boons for being an evangelist of nethys are:
1: Arcane Essence (Sp) mage armor 3/day, mirror image 2/day, or fly 1/day
2: Arcane Eye (Sp) You can use arcane eye three times per day as a spell-like ability. The arcane eye you summon functions as if you had cast arcane sight and were able to view its information through the arcane eye. This allows you to see magical auras through the arcane eye, and potentially identify the schools of magic involved. You can also potentially determine the spellcasting or spell-like abilities of viewed creatures, as noted in the spell description.
3: Robes of Nethys (Su) You can manifest an illusory robe that absorbs hostile spells for a number of rounds per day equal to 1 + 1 for every 4 Hit Dice you possess (maximum 6 rounds). These rounds don’t need to be consecutive, and you can activate or deactivate the robe as a free action. The robe functions as a lesser globe of invulnerability except that it only excludes hostile spell effects of 3rd level or lower. Any spell that would force you to attempt a saving throw; cause you to take hit point damage, negative levels, ability drain, or ability damage; or end your life is considered hostile for the purposes of this effect. Unlike a lesser globe of invulnerability, you can move normally while cloaked in the robes.
Inner Sea Gods pg. 100
Best Answer
There is very little reason not to enter the prestige class as soon as possible. BAB matters not even slightly for the overwhelming majority of wizards: touch attacks are easy to make, and a typical wizard should never be targeting armored AC (or caring about iteratives). The saving throw bonuses matter somewhat more, but ultimately you’ll never be down by more than 1; valuable, but not worth delaying class features over.
Unfortunately, I should warn you that Paizo has overcompensated for what they felt was too much use of prestige classes in 3.5. The game punishes you harshly for any multiclassing, and that includes prestige classes. I don’t own Inner Sea Gods, but unless these divine boons are phenomenally potent, evangelist is actually a pretty harsh downgrade in power. Wizards are overpowered, and PFS play is not typically all that demanding, power-wise, so you’ll probably be fine even with a downgrade, but you should go into that aware of it.