I was looking at the feat-trees associated with each, and was kinda miffed that you could get Great Throw (Impr. Trip) much earlier than Grappling Block (Impr. Disarm) making both of the feats available for choice kinda valuable at lower levels. I know any monk I will build will probably be taking one of them before reaching 6th level, leaving the other as the 'ok, I guess I will take it, I have no choice now' feat.
For example you could take some levels in fighter to climb the feat tree quicker, getting Grappling Block by PC level 6. Or you could get Great Throw by 3rd (I think... not exactly sure though).
My monk-mage will be getting Grappling Block by 6th level/3rd monk (if he survives) and will still be 3 levels short of getting the 6th level monk feats. And at 9th level/6th level monk he will be ready to get Impr. Trip and the bonus feat, getting Great Throw all in one sweep just because I had to take Impr Trip.
To me, I would have writen that the monk gets to make the choice between these two feats at an earlier level, say 4th, and give two more feats at 8th. With the automatic feats at 3rd and 6th it would even out so that the PC gains them in a slow progression, kinda like the fighter.
As far as worth, I would say both have their place. Having Improved Trip (or better yet, Great Throw) would be great when you are fighting foes on cliffs, tops of walls, bridges, etc. Improved Disarm (or Grappling Block) is wonderful for that evil melee guy who thinks he is all powerful... that is until he has to draw his second weapon out... followed by his third...then resorting to shield-bashing the annoying monk.