Back in the NiCad days, we liked to discharge at a high rate because NiCads seemed to perform better when discharged that way (less tendency to feel "flat" near the end of a run.)
NIMH cells apparently don't exhibit this memory effect, thus I don't think that it matters whether you go for a 20 or 30 amp discharge after a run. I try to choose a discharge rate that will be approximately the same as what the car sees on the track, so I can time the discharge after the run to get an estimate of how much runtime was left in the pack. For 1/12 scale, we run 8 minutes, which would average approximately 24 amps during the race if we dump jsut after the heat is over. By discharging at 24 amps right after the race I get an idea if I had enough runtime left to go to a hotter motor or gearing setup.
For 5 minute touring car racing, I'd discharge at 38 amps if I could, again just to get an approximation of true remaining runtime. My Victor won't go that high, so I settle for 35 amps.
If you don't care to get as compulsive as I do, I think either a 20 or 30 amp Reactor will do the job just fine. If I were racing touring cars, I'd go with the 30, but again, I'd go that route to get a better idea of remaining runtime, not because I feel that 30 amp discharges benefit NIMH cells more than 20 amp.