Never heard of Mailloop. Getresponse from what I hear is very similar to Aweber, although not as good as far as ease of use and message delivery. I'd say Aweber is the way to go.
I have no experience with Mailloop, but I have tried GetResponse a while ago, and I am a current member of AWeber (been for more than 3 years). I'd go with AWeber, it's plain awesome. Their support is fast and accurate, their service is powerful and easy to use, and their email deliverability is the best (your email reaches the inbox). - jens -
Aweber beats everyone in game but getresponse is also goodone. But if you compare both services Aweber come out as winner in speed, interference and accuracy.