I've done the RR but not the crit. The crit is flat. Use flat gearing.
If you do the RR or the curcuit race, I used a 39 - 27. On the RR there is the wall, it's about 250' in about a tenth of a mile. You have to do it about 4 or 5 times, depending on what cat you enter. I've seen people use a 23, but they are younger and have stronger knees... I prefer to "spin", but if you can spin in a 27 up either the rr or cr climbs, you're stronger than I. The RR ends with a 2 mile 700' climb out of the valley up over the hill to the track. That's where the race is won. Hang in there and save as much as possible for this climb.
Good luck.