To fate only vs mob grind + fate

Today I got into a disagreement on if only getting xp from fates equated ~ the same amount of xp vs grinding mobs in between fates and also getting xp from fates. This was in Northren Thanalan where it seems many fate parties have chosen to mass kill the grenades and followers in between fate spawns. I have always chosen not to do the latter because to me there seems no real BIG difference in the short term between the two options. Could some one show test this out? Also does the chain bonus % continue to increase the higher the chain or does it cap off at 50% even if you get a chain of 20 to happen?

It's about 8,000 XP if you kill that whole group in N. Thanalan, and half the time there's no other FATE up. It's definitely worth it between FATEs.