The specialist wizard is in one way weaker than the generalist wizard: they are barred access to 2 schools of magic (maybe one if they are diviners). It is generally (but certainly not universally) accepted that the specialist is more powerful than the generalist overall (I've seen designers point this out as a legacy imbalance from 2e; makes sense to me), although certainly not as versatile alone. This lies in the fact that you get to choose your specialisation and your barred schools. Assuming that you choose a specialisation that fits your character concept correctly, you will always want to prepare 1 spell per spell level from your specialty each day, so the requirement for the bonus slot to be from the school is nearly meaningless (I have met *one* wizard who felt chafed at all by this requirement, and he turned out to have specialised to reach a prestige class and not because he liked the school). That said, let's look at an 18 Int wizard (I would just use the base chart to illiustrate my point, but that would be unfair: most wizards eventually get 18 Int, and less Int makes this very bad for the generalist very quickly) and how many spells per day he gains from specialising (for simplicity's sake, I have converted spells into their SP equivalent so I can add them all up, counting cantrips as 1/2). Gain is the ratio of specialist to generalist. Notice how the generalist falls over a level behind the specialist starting at level 7, and it goes downhill very very quickly from there. In fact, after 14, the generalist falls over 2 levels behind the specialist!
Level 1: Generalist-3.5, Specialist-5, Gain-1.42857
Level 2: Generalist-5, Specialist-6.5, Gain-1.3
Level 3: Generalist-11, Specialist-15.5, Gain-1.409
Level 4: Generalist-15, Specialist-19.5, Gain-1.3
Level 5: Generalist-25, Specialist-34.5, Gain-1.38
Level 6: Generalist-33, Specialist-42.5, Gain-1.2878
Level 7: Generalist-48, Specialist-64.5, Gain-1.34375
Level 8: Generalist-60, Specialist-76.5, Gain-1.275
Level 9: Generalist-72, Specialist-97.5, Gain-1.35416
Level 10: Generalist-88, Specialist-113.5, Gain-1.2897
Level 11: Generalist-104, Specialist-140.5, Gain-1.35096
Level 12: Generalist-124, Specialist-160.5, Gain-1.29435
Level 13: Generalist-144, Specialist-193.5, Gain-1.34375
Level 14: Generalist-168, Specialist-217.5, Gain-1.29464
Level 15: Generalist-192, Specialist-256.5, Gain-1.33593
Level 16: Generalist-220, Specialist-284.5, Gain-1.29318
Level 17: Generalist-248, Specialist-329.5, Gain-1.32862
Level 18: Generalist-280, Specialist-361.5, Gain-1.29107
After this, the wizard spell progression breaks the usual pattern, plus most campaigns probably don't go this far, so I'm stopping. It's worth noting the general ratio is usually about 1.3ish, so the specialist might last four encounters, while the generalist runs out of spells after three. Sometimes, this advantage isn't worth losing two schools, but more often than not, it is. At low levels, the ratio is larger, and at high levels, the gap is so wide that the generalist is multiple levels behind. Specifically, depending on the PC's moral qualms with certain spells, it is often not too awful to lose Enchantment and Necromancy (there are some really great spells here, but nothing whose role can't be covered by other schools or your friendly divine spellcaster). That said, parties are often large enough to have a bit of overlap, which can help in deciding your prohibited schools (I'd feel even safer losing Enchantment if my party had a Bard, for instance). Some of the best 6-person parties I've seen have two specialists who have prohibited themselves from each other's specialty schools (especially back in 3.0 when you could have only 1 prohibited school sometimes). Nonetheless, sometimes a generalist is the right person for the job, I've just never had a player select one, even once, playing from AD&D 2e-3.5.
