In several threads here on ENWorld, I've expressed my dislike of the Mystic Theurge-style casting PrCls, but everything up to this point has been opinion or "feel." Until yesterday, I hadn't hit upon a way to quantify my reasons.
The problem is that some spells in a given level are more or less powerful than others of the same level, or have effects that are not measured in hard & fast numbers for comparison. Then there is the question of class features.
So, we need to find a way to find a common basis for comparison across all spells. The closest we come is the Heritage Feats, several of which have assigned a value of 2D6/spell level to all arcane spell slots. IOW, a 1st level spell is worth 2d6 damage. A 3rd level spell is worth 6d6 damage. Etc.
If we then apply this value to each spell a PC of a given level can cast, we find out how many d6 of damage that PC can do in a 24hr period- IOW, what happens if he "goes Nova." Lets call this total his Spell Power Rating, or SPR. Obviously, its not a perfect metric- all of the Heritage feats that grant this type of ability are Area Effect attacks, and we have no way to calculate how many targets get hit, how many made their saves, etc. This is, thus, just a rough baseline comparison.
Ignoring bonus spell slots gained from high characteristics, feats or magic items, we get this:
Wiz20: SPR = 360d6; Avg Nova dmg = 1260hp; Max Nova dmg = 2160hp.
SpecWiz20: SPR = 450d6; Avg Nova dmg = 1575hp; Max Nova dmg = 2700hp.
Sorc20: SPR = 540d6; Avg Nova dmg =1890hp; Max Nova dmg = 3240hp
The "worst case scenario" arcane caster combo
Sorc5/FocSpecWiz5/Ultimate Magus10: SPR = 714d6; Avg Nova dmg =2499hp; Max Nova dmg = 4284hp
Thus we see that the Sorc5/FocSpecWiz5/Ultimate Magus10 does 2x more damage on average when going Nova than a Wiz20 does, and 15% more damage than the Wiz20 maxing out. It also does 33% more damage when maxing out than its nearest competitor doing likewise.
Now, of course as stated before, the SPR does not take into account things like the overall flexibility of the Wizard who has no upper limit on spell selections and a host of bonus feats, the situational flexibility of the Sorcerer who, despite knowing fewer spells, can choose what he wants to cast in real time, or the big difference in the quality of spells as their level increases.
Sure, there are situations where you simply can't solve a problem by slinging damage at it, and that is where the higher level spells earn their keep, and why so many players (not me) bemoan the cost of multiclassing for spellcasters.
But while it isn't perfect, SPR does provide one truly quantifiable basis for comparison.
The problem is that some spells in a given level are more or less powerful than others of the same level, or have effects that are not measured in hard & fast numbers for comparison. Then there is the question of class features.
So, we need to find a way to find a common basis for comparison across all spells. The closest we come is the Heritage Feats, several of which have assigned a value of 2D6/spell level to all arcane spell slots. IOW, a 1st level spell is worth 2d6 damage. A 3rd level spell is worth 6d6 damage. Etc.
If we then apply this value to each spell a PC of a given level can cast, we find out how many d6 of damage that PC can do in a 24hr period- IOW, what happens if he "goes Nova." Lets call this total his Spell Power Rating, or SPR. Obviously, its not a perfect metric- all of the Heritage feats that grant this type of ability are Area Effect attacks, and we have no way to calculate how many targets get hit, how many made their saves, etc. This is, thus, just a rough baseline comparison.
Ignoring bonus spell slots gained from high characteristics, feats or magic items, we get this:
Wiz20: SPR = 360d6; Avg Nova dmg = 1260hp; Max Nova dmg = 2160hp.
SpecWiz20: SPR = 450d6; Avg Nova dmg = 1575hp; Max Nova dmg = 2700hp.
Sorc20: SPR = 540d6; Avg Nova dmg =1890hp; Max Nova dmg = 3240hp
The "worst case scenario" arcane caster combo
Sorc5/FocSpecWiz5/Ultimate Magus10: SPR = 714d6; Avg Nova dmg =2499hp; Max Nova dmg = 4284hp
Thus we see that the Sorc5/FocSpecWiz5/Ultimate Magus10 does 2x more damage on average when going Nova than a Wiz20 does, and 15% more damage than the Wiz20 maxing out. It also does 33% more damage when maxing out than its nearest competitor doing likewise.
Now, of course as stated before, the SPR does not take into account things like the overall flexibility of the Wizard who has no upper limit on spell selections and a host of bonus feats, the situational flexibility of the Sorcerer who, despite knowing fewer spells, can choose what he wants to cast in real time, or the big difference in the quality of spells as their level increases.
Sure, there are situations where you simply can't solve a problem by slinging damage at it, and that is where the higher level spells earn their keep, and why so many players (not me) bemoan the cost of multiclassing for spellcasters.
But while it isn't perfect, SPR does provide one truly quantifiable basis for comparison.