There's merit in both camps -- the simplest way to resolve the whole discussion would be to revise the rules such that trip attacks can't be used as Attacks of Opportunity.
For myself, I prefer to rule that the trip attack on rising from prone results in a prone character, action wasted. Here's why I look at it that way:
(1) AoOs occur during the action that provokes them ( I don't see any evidence that they occur before them, except from interpretation of some of the special attack forms, where the AoO is the first thing resolved, but that doesn't indicate to me that "the AoO takes place before the action").
(2) It's clear that AoO's can interrupt or prevent some actions -- specifically (1) spells, subject to a Concentration check, and (2) grapple attempts. So there's precedent there. On the other hand, AoO's don't prevent other actions from occuring -- like drinking a potion, or firing a ranged weapon, or moving out of a threatened square. There's the rub -- if the action is not specifically mentioned, which category does it fall into? The safe bet is to say the action continues unless the rules specifically say it doesn't, as with a spell or grapple -- but
(3) Trip changes the target's state from standing to prone, and hence is itself an interruption. So a succesful trip attempt must change the target's state from "attempting to stand" back to prone, with the resulting loss of action.
So as I see it, it can be interpreted either way with roughly equal validity. What reinforces my view of giving the advantage to the tripper is this: the standing character has a distinct advantage over the prone character, and that includes being able to prevent the prone character from standing. It should, IMO, be possible for an attacker to keep a defender prone indefinitely, provided that (1) the attacker is competent enough, and (2) all the defender keeps doing is trying to get up. IMO, there's no god-given right to stand up from the prone.
If someone wants to optimize a character to be a trip-monster, more power to him; it isn't any worse in my experience than any other focused power-gaming tactic.
Beware the one-trick pony, though, 'cause someone else will come up and eat the bully's lunch while he focuses on keeping his victim on the floor.