Improved Trip vs. Stand Still

[Yorae]

I've seen many lockdown builds take both of these abilities and I was wondering why.

What is the point of halting an enemy via stand still with your AoO, when you could instead use that AoO to trip that same enemy (and get a free extra attack). Is there some synergy I'm not seeing here, or should a lockdown build take one of these feats and ignore the other?

[Glimbur]

Large or Huge or bigger foes have large bonuses to resist trips, but their reflex saves tend to be poor. Therefore Stand Still is a back-up if it seems unlikely that tripping will work.

A few things are entirely immune to tripping, but their number is small.

[Ernir]

Yeah, not so much a synergy as it is a backup.

If you can trip, it's better to trip. But you can Stand Still in more situations than you can trip.

[Eldariel]

Also, you can't trip an opponent standing up (again) but you can Stand Still them with the AoO to make them unable to go anywhere anywayz (only to get tripped again next turn).

[olentu]

Also, you can't trip an opponent standing up (again) but you can Stand Still them with the AoO to make them unable to go anywhere anywayz (only to get tripped again next turn).

I think stand still is only for movement out of a square.

[Eldariel]

I think stand still is only for movement out of a square.

Right. It wasn't just any move action that provokes. Well, oops. IGNORE ME!

[MeeposFire]

Improved trip is not always a good option since it has limitations such as size or immunities. Stand still always works if they fail a nasty save which is more important for a lockdown, though of course you trip if it is an effective option against a specific enemy. Remember tripping builds have glaring holes so relying on it as your primary ability is not a good idea unless you are stuck at core only.

[Lateral]

Also, remember that tripping is more feat intensive; it requires two feats (or two Barbarian or Monk levels, I guess) to work versus Stand Still's one, and it requires three to be really good (Knock-Down). If the build is already very feat-starved (which, if you're talking about that Ur-Vindicator build, it is), you probably don't want to take tripping on top of that.