Levels in Tunnels & Trolls don't have the same meaning in Tunnels & Trolls as they do in Dungeons & Dragons. In all of D&D's various incarnations, level gain means an increase in Hit Points, additional spells memorized for the spell casters, possibly a new level of spells to cast, possible increases in THACO and Saving Throws, turning undead increases for clerics, increase in thief abilities and the rest of the cornucopia of abilities that D&D characters posses.
In Tunnels & Trolls, gaining a level has much fewer benefits for most characters. In 5.5e and earlier, gaining a level results in a stat increase (in 7.x, stats are increased with AP earned, and stats decide character level). For Rogues and Wizards, it gives access to higher level spells. For Wizards, it can decrease the cost of casting lower level spells. Oh, and 7.x also gives Warriors a +1 bump per level to their Combat Adds.
I personally don't like deriving character level from stat number. I do, however, like the idea of spending 10x current stat to increase the stat by one.
So, I figure I'll use the stat increase method from 7.5e (7e is 100x stat... way too harsh), while keeping the total AP earned to determine character level.
Not a bad compromise IMHO.