They all seemed about the same to me they just don't last long enough. I quit using them but our fly season lasts 9 months. If I had just a few cows, I would change them out regularly but I don't.
Blacksnake is correct in that it is almost impossible to tell how good they work from year to year with different fly concentrations. Rain can make a lot of difference in the amount of the pests.
When I used them, the XP's worked good for a couple months.