Sorry to bump an older thread, just want to give my input.
I've used both services and like anything, they both have pro's and con's.
Aweber
Pro's - Long time service provider, you can trust they will probably be around a long time. Huge knowledge base and their support is very knowledgeable about the product. Never had a question not get answered properly or problem unsolved. Friendly support as well.
Con's - Templates are quite outdated. Some down right ugly for these days. RSS to email option is lacking. Quite strict on using double opt-in. Though they have recently changed somewhat on that topic.
GetResponse
Pro's - Little cheaper than aweber. More single opt-in friendly. Ton of very nice templates, many modern and stylish. Fantastic RSS to email feature, even have email templates just for that purpose. More options than Weber overall.
Con's - Several years back I used them and the support personal was not very knowledgeable. Kind of brushed me off a couple times. I am trying them again and it seems they have improved greatly, just haven't "tested" them yet to know for sure.
Hope that helps.
