Redbook Audio Aero Prima vs SACD palyer?

It depends on which SACD player you're talking about. Of course, if you're going to drop a couple of grand on an SACD player, it'll probably be a good one. The Prima has a pretty good reputation around here for redbook performance, but it can't do SACD. Just make sure you're SACD player can do redbook CD as well as the Prima.
But then, do you really plan on dropping a lot of coin on SACDs? They are expensive and the selection isn't nearly as large as it is for CDs.

Golden_ears- I agree with everything you said besides the SACD's being expensive, I haven't purchased a SACD for over $16.99, sure some of the reference Mo-fi and analogue productions stuff retails for $25(which still is not bad for how good they do sound, I have a few of each) but you can usually always find them used for around $15ea. The ones that I am unable to find I am near my breaking point and am going to spend what it takes to get them. All of that being said retail price on CD's has just come down which is the only reason there is a discernable difference between the two format's as far as price is concerned. If you just impulse buy SACD's I bet it could be expensive, but so can regular CD's retailing for $18.99 in some stores still!

You're idea is not far-fetched. I find that my redbook performance equals my SACD performance in most cases.
You will need a DAC to get the best of both worlds if you want true premium SACD and redbook. I use an XA-777ES for SACD, and the TRUTH is that I could live without the SACD if I were playing 24 bit remastered redbook CDs via my ECD-1 DAC. There are a few exceptions, but they are very (too)few at this point.