Hi Paula,
My first comment on any of the forums and this one caught my eye.
Personally I've used both Aweber and Get Response and from my perspective personally, both are pretty much equal both from a deliverability and functionality perspective. In terms of deliverability, I think you'll find when you do your testing that we're talking 10ths of a percentage point either way on an ongoing basis.
In my mind they're pretty much neck-in-neck with my preference being GetResponse. GetReponse is certainly less expensive ($ 15.00/month for up 1,000 subscribers as opposed to $ 19.00/month for up to 500) and has a marginally better affiliate pay-out (33% to 30% through Aweber. The other thing that may sound really trivial, is that GetResponse does offer a free 30 day free trial (I know it may sound silly, but the whole charge $ 1.00 for the trial thing is a pet peeve of mine).
I've also user TrafficWave, GVO and Global NPN's offerings. All three of these are certainly almost comparable in many ways and not bad options either.
I'm personally using All In One Profits right now and their AIOP Response product. I will admit, it's not anywhere near as feature rich as the others mentioned here, but for what I need right now, it does the job. Over the last couple of months I haven't seen any significant issues related to deliverability and the price is certainly right. Similar to TrafficWave you get unlimited subscribers and unlimited emails. The cost of AIOP response is included as part of the subscription to AIOP which includes a pile of extra functionality. So all in all, while I'm sacrificing some feature richness that I'm just not using now, I more than make up for it at the price point, with no measurable lack of deliverability at this point.
Hope this helps...