Samurai said:The difference is a Theurge only goes for 10 levels. That means that by 20th level, the character has only 10 double-caster progressions, and 10 singlecaster progressions.
A True Nec works out exactly the same because it's a 14 level PrC... 10 single caster progressions and 10 double caster progressions, plus powerful class abilities at every level.
If Mystic Theurge continued for 14 levels, you'd have a point... then, at 20th level, the Theurge would be a 17/17 caster, while the True Nec only a 15/15 caster. Do you allow characters to keep advancing past 10th level as a Theurge as a house rule?
I'd have to see the class, but if the TN can't go past 15/15 split, and only gets a few 9th level spell-like abilities at high levels, then it doesn't sound overpowered at all. Using cumulative spell levels is not a good way to assess power, because it doesn't take account of time. If both a 9th level and fifth level spell cast in a standard action, the 9 spell levels are worth more than the 10 spell levels from two 5th levels because it only took one round to cast the former as oppossed to two. And given the short front loaded nature of most 3.x combats, you might very well not get to cast that second fifth level in any case.
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