If you want the Amazon exclusive Aston Martin Virage, get it through amazon. But keep in mind that in about 6-7 months, all the exclusive pre-order cars will probably come out in a launch pack.
Microsoft's car is the Nissan 370Z which also comes with other retailers so its less exclusive. If you can wait for the Aston Martin, get it though Microsoft. You can save the 1600MP ($20 value) for later and buy all the pre-order cars later on. You cal also use the $10 coupon on the $50 (4000MP) Season pass if you're up for that so it'd be slightly less ridiculous.
thanks L. your post makes a lot of sense , sort of along the lines of what I was thinking about. gonna probably go through microsoft store. amazon is great but they haven't had as many good deals as they have in years past.
one thing that kinda turned me off and made me ask this question was amazon not having free release date delivery. since I couldn't get that I figured I might as well just try to get the best deal on the game.