Kraken for the ability to wire money directly from bank, and poloniex because of pair selection. Polo has a good selection of alts, what I'd call 1st tier coins.Any reason why you chose those two over others?
I haven't traded on kraken recently but the new interface I tried a few months ago was miles ahead of the competition, however, it was in beta and a glitch cost me some potential gain, and I haven't been back yet to check out the finished product.
The exchange decision comes down to what pairs you want to trade, how you want to trade them (volume), and withdraw limits.
If you're day trading, you want big volume, but if you're first in line on an obscure alt coin with a big future you may have to use a lesser exchange.
Always know limits before you move money in, as some have very low (2k a day and lower) withdrawal rates for unverified accts...and you don't want to be stuck moving a few hundred thousand 2k at a time.....i mean what a great problem to have and all but around here we're trying to eat the cake too.