Personally I have very little experience with Aweber and most experience with GetResponse. Functionality wise I believe they are very close to the same and, in most cases, each will cover the needs we all have. The extra goodies that aren't "needed" are additional in my eyes. But, probably like most here, I don't need most of the functions.
Cost wise I do believe GetResponse is lower. When you have tens of thousands of subscribers, the cost can certainly add up either way. As long as deliverability is normal or better, the necessary functions are at my finger tips and I'm not spending more than required, I'm a happy camper (generally).