The loss is that RC only gives 40.5% (think thats it) of a rune, where as RE gives 45% of a rune. So in terms of maximum output RE wins, but RC will give you a smoother rotation but less runes, which is nice but again RE will give more runes.
Yes that's exactly right. If you really enjoy RC more than RE, go ahead and use it, the difference is quite minor for 2H frost. The only spec that's locked in is DW frost which must use RE, as it's integral to masterfrost.