Are the rafters paddling or rowing? Did you want to do an overnight or just camp by the river? Both the Ark and the Poudre have great class 3 and class 4 runs. You can do about 10 straight miles on the Poudre and have quite a few class 4 rapids, camp and run a different section the next day with a couple of other class 4's on it. On the Ark you could run Brown's Canyon, which is almost all class 3 in the first day and see how you feel about Numbers for the second day, though it's pretty stiff class 4 at higher flows.
Of course there are plenty of good rivers to run in the state, and some might be better in mid-May than these two, but the Arkansas and the Poudre have great class 3 and 4, are easy to shuttle on, and offer great camping spots. If you let me know what you decide, I can give you some good camping beta.