QGD: Exchange variation move order

If I want to play the QGD exchange variation is there any reason I shouldn't exchange on move 3 (ie 1. d4 d5 2. c4 e6 3. cxd5) instead of move 4 (ie 1.d4 d5 2.c4 e6 3. Nc3 Nf6 4. cxd5)? Why give black extra options on move 3 when you can force an exchange variation on move 3?

well I would think you would play it right away just to avoid a tarrasch, as in any case in the exchange white would want a N on c3 and black on f6 anyways.

I cant see much of a difference, it would be interesting to see what others think

You may possibly want to chek from database or opening books but I think the following setup might just be interesting for black. Possibly somewhat more flexible than the normal variations.