The RC-3600 HV should have lower internal resistance - get that one over the the nicad one.
However, if you're willing to spend the money, get Intellect Brothers Sub-C cells instead.
http://www.onlybatterypacks.com/show...temID=10221.21
Or if you need 4/5A size, Gold Peak is a good bet.
http://www.onlybatterypacks.com/show...temID=10215.21