I've used both Aweber and GetResponse. I'm in the internet marketing niche, where most people know about Aweber. So I have few problems with getting people to double opt-in. Everybody knows the way Aweber works, so getting subscribers seems to be easier with them.
Plus, I like all the different subscription form templates that Aweber has. And lastly, because Aweber has been around since forever, they tend to have a great rate of deliverability. I do find a few Aweber emails in my spam box, but not as many as other providers.