I think the "DC 15" (or 3.0 D&D) version makes tumbling too strong, and the "opposed attack roll" (or Monte Cook) version isn't appealing in that it, in effect, makes Tumble just another form of AC that applies to attacks of opportunity and has almost nothing to do with your action AC. It's also true that it's often the case that opponent's attack bonuses will outstrip tumbling bonuses, which might make tumbling too weak.
Instead, I go for a happy medium. My system is motivated on the following underlying assumption: tumbling through a threatened area should make you better at avoiding attacks of opportunity that your movement provokes. To me, "better at avoiding attacks" is most simply and effectively represented with AC bonuses. Since my system can provide very large bonuses to AC, tumbling continues to be useful, but you need to be very good at tumbling if you want to avoid attacks of opportunity from foes who would normally hit you most of the time (which, I think, is the way it should be). It also retains the value of the Mobility feat for tumbling characters, which I think is a good thing to do. But without further ado:
a. When tumbling through a threatened area, you get a circumstance modifier to your AC versus any attacks of opportunity provoked by your movement equal to (half your skill check) - 5. (Tumbling really ineptly actually makes you easier to hit, as you're paying less attention to dodging). Thus, a skill check result of 19 gives me a +4 AC versus attacks of opportunity.
b. Tumbling through an occupied space works the same way, but the circumstance modifier to AC becomes (half your skill check) - 8. Thus, a skill check result of 19 gives me a +1 AC bonus in this case. Characters always threaten the space they occupy, even if they don't threaten adjacent spaces (if they're wielding a reach weapon or a ranged weapon). If you don't threaten an adjacent space with your weapon, you make your attack of opportunity with an unarmed strike or other natural weapon. If the attack of opportunity hits, my move action ends immediately, in the square I can through just before I tried to tumble through the occupied space.
