First, let me say that some seem to be treating SPR as if I'm asserting that it should be the ONLY consideration. I'm not.
(2nd, this post is going to be incomplete again- RW time pressures.)
I'm putting it forward as a
single metric, just like a baseball player has a batting average, his RBI, a slugging percentage, a RISP rating, an on base percentage, WPA, WPA/LI, Clutch (and probably a few other stats I'm probably forgetting) to measure his value as an offensive force on the diamond.
SR is a significant factor, as are saves, especially if you are comparing, as you are, pure damage.
You're missing the point.
If you look at spells, some have saves, some don't, and there are 3 different kinds of save types. Some are affected by SR, some aren't. Some spells target foes, some allies, some only target you. Some spells are poorly designed and could be considered suboptimal for a given level, others are quite well designed and are arguably designated at a level lower than they should be. Sometimes there are XP costs associated with casting a spell, long casting times, unusually long durations , etc.
That is a
lot of variables, making comparison of them head to head virtually impossible.
What filtering all of those spell slots through a Feat like Dragon Breath does is remove virtually all of those variables, leaving only the damage dice. All other variables drop out.
Choosing to ignore as you do, the specialist wiard having actually 5 of each level at 20th level, and the effect of the theurge having to diversify his casting Stats, as opposed to focusing in one stat.
Actually, I originally posted with the
Focused Specialist, who has 6 of each spell.
And if you have a problem with diversity of stats, substitute the Beguiler class who has virtually the same progression as the Sorcerer (only losing 1 9th level spell) and still has Int as a casting stat, while remaining a spontaneous caster.
I explicitly left out bonus spells of the equation because:
1) Quality of stats will vary depending upon the method used in the character generation process. A PC with high starting scores in both casting stats would only have a difference of a couple of slots by Lvl 20, not enough to make a significant difference.
I doubted your numbers, so I double checked the Ultimate Magus as described in the Complete Mage, Please correct me if my understanding is incorrect:
The ultimate magus (5/5/10) will have the following actual caster levels:
5+9= 14!
5+8= 13!
To meet the prereqs, a PC could be Sorc1/Wiz3. The UM lets you add to the lowest one, giving us (effectively) Sorc2/Wiz3 at the first UM level. At 4th, Sorc pulls even, and at 7th, pulls ahead (since there is no "lowest" class, its player's choice). Every other level, both classes get bumped.
So its 5+10 = 15; 5 + 7 = 12
(I'm not saying I didn't miscalculate spell slots though- its easy to make mistakes when you're doing this kind of thing on your PDA.)
Don't forget:
1) The UM class has a built in "Practiced Manifester+" effect, casting arcane spells as if he had that feat for both of his classes.
2) I obviously assumed that there would be a balanced approach to taking levels. If, however, you do an unbalanced advancement, you get Sorc7/Wiz3/UM10, which gets you back up.
3) I'm not ignoring the value of an 8th level spell vs a 9th level spell. However, you also have to account that some 8th level spells are nearly as good as average 9th level spells, and some 9th level spells are probably overpriced and should be 8th level spells.
That returns us to why I went through the Feat- it takes spell quality out of the equation.