The CAAD8 (and CAAD9) have a geometry that is a bit more race-oriented than the Allez, but not too much different. (If you think you'd like an even more upright position than even the Allez, Specialized also makes the Secteur line. You can somewhat view the Allez as the aluminum version of a Tarmac, whereas the Sectuer is the aluminum version of a Roubaix.
(Cannondale's more relaxed geometry is the Synapse line).
You might find that with comparable component equipages the Cannondale bikes are slightly less money than the Specialized bikes, but generally speaking the equipages will be very similar across brands, at comparable price points.
In addition to the CAAD being somewhat racier than the Allez, I think you will find that it is a bit lighter, also. But in the end, the bike that feels better will be a better choice.
It's also true that for a test ride you are only going to get a couple of adjustments made to "fit" the bike to you, even if they have the right frame size (i.e., probably just setting the seat height roughly right), whereas a real fitting on a purchased bike will also look at saddle fore-aft position, saddle tilt, possible stem adjustments, etc. So you might not want to reject a bike just because the fit isn't quite right during the test ride.
As for components, I am guessing that both are Shimano 105/Tiagra blends? Probably both are using "other brand" brakes. Not to tell you things that you already know, but the Shimano "pecking order" from cheapest to best is:
2200, Sora, Tiagra, 105, Ultegra, Dura-Ace.
So more 105 and less Tiagra (and less Sora) is good. Something to keep in mind is that the 105 line is just recently (now) being redesigned a bit - but I don't know how long it will be until new bikes using 105 start using the new version (the old 105 has part numbers beginning with 56, whereas the new 105 will have part numbers beginning with 57).
I don't think the differences will be that much, but it's at least something to be aware of.
I THINK the biggest area of difference is in the brake/shift levers (the new 105 will have "hidden" derailleur cables)- but at your price range you may be talking Tiagra levers anyway.