Join Date: Oct 2006
Location: United States of Mexico
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I think suspension seat posts are junk for a variety of reasons, especially the cheap ones. Same goes for adjustible stems. They're gimmicks that compensate for a poorly designed bike.
I also think that an aluminum frame of any sort would be the worst possible thing for your back, even with the gimmicky add ons.
Your focus should be on two things. First, research bike geometry and find a brand or line that will allow you to ride in a position that works with your ailment. This means doing a lot of longer test rides. Dial in the stem/handlebar position from there. Second, look into a steel frame (Bianchi, Lemond, Gunnar, Surly etc.) or, if you have some cash, get a carbon fiber frame. You want something compliant that doesn't communicate high frequency road vibrations.
After you get the right frame, you can add things that will help reduce those vibrations even further like saddles with ti rails, cf seatposts, larger tires.
Don't get caught up in componentry, your back problem means that you need to focus on finding the right frame.