Have used both in the past. Both have awesome quality, but they each have different advantages.
Rattlerrepros has better prices, faster shipping and more different repro sticker sheets available, but you have to cut them out yourself.
CobraStickers has more faction stickers (like Star Brigade logos, country flags, rank emblem, etc) and they are all pre-cut, but you pay a little more and they take longer to receive (which is surprising since they are in Alberta).
OB