Mosop has run 3 marathons:
2nd at Boston 11 in 2:03:02 (way under old CR)
1st at Chicago 11 in 2:05:37 (CR)
3rd at Rotterdam 12 in 2:05:03
Also of note is Mosop's 25k and 30k world records.
Kirui has run 9:
9th at Berlin 06 in 2:17:47
1st at Vienna 08 in 2:07:38 (CR)
2nd at Berlin 08 in 2:06:51
3rd at Rotterdam 09 in 2:05:04
1st at WC 09 (against a field weaker than any marathon major) in 2:06:54
5th at London 09 in 2:08:04.
DNF at London 10
1st at WC 11 (once again, weaker field than any marathon major) in 2:07:38
6th at London 11 in 2:07:56
In their one race together, Kirui attempted to get the 25k world record with Mosop going for the 30k, but Kirui couldn't hang with Mosop (ie, got absolutely destroyed) even though Mosop was running 20% farther.
At the end of the day, Kirui hasn't broken 2:06:54 in over 3 years and has only gotten wins against weak fields at world championship marathons and Vienna. Say what you want about Mosop's run at Boston with the wind, but he is clearly able to run 2:05s without a problem and has been consistently very fast in all 3 starts. Kirui is nowhere near that level right now and he blows up in most of his races. In 9 starts, he has 3 finishes that I could see people arguing as being excellent (Rotterdam 09, WC 09, and WC 11, though I would argue against the latter 2).
My prediction for August: Abel Kirui, who is clearly vastly inferior to the other 2 on the Kenyan squad, will get eaten up not only by Kipsang and Mosop at the Olympic Marathon, but also by the Ethiopians and even possibly a few others.