There is no real hard numbers that I have found, but the harrington and neilson provide the highest buffs. There are tribbles that provide multiple buffs but you have to acquire those either through best buy preorder, refer a friend or (i believe) the borg tribble from the tribble weekend event on the test shard a couple months back.
Also, I am Klingon on fed side, so tribbles don't like me. The photonic tribble is the only one that buffs me. I think, if the buffs are provided you can watch your stats and apply the buffs and see what stats change and from that derive which is better. According to the Tribble wiki page, the harrington and neilson provide the best damage buffs while the other two you have mentioned provide a lesser buff.
Have a look at this thread and its link, a very nice graphical overview of how to breed the various Tribbles. Al though the flowchart doesn't cover the actual stats it gives you a nice overview of the different strengths of all the Tribbles.