Both could get quite large in the typical reef tank. I'd stay away from puffers in reef setting especially there are fish smaller enough to be its appetizers. Cow fish, due to their mouth size and slow growth rate, are more suitable for reef tank. I've had several cowfish and I've yet experienced them nuking the tank at death. I've had cowfish that died in my tank, and I have not witnessed the phenomenom. Spiney puffers, on the other hand grow very fast.
I had the spiny puffer back when the tank was FO. It was kept with triggers, snowflake, and lionfish. At the time, it was fed with feeder crab, fresh seafood, krill and occassional treat of earthworms. It quickly grow from 1" body size to over 6" in 18 months.