If you have base 2.73/auto and you demand to keep your warantee, it is worth every penny for 5.0, as the base 2.73/auto/V6 is a total dog out of the hole. The AWD fusion (2.0 ECO) I just got as a daily would dog the crap out my v6 stang across an intersection, easy (after that obviously stang would blow on by).
If you have a 3.31 gear (or perf package), will not be nearly the difference and may not be worth the loss of money and mileage over the V6.
Also, if not that worried about warrantee, you could put a set of 3.55's in and I'd guess the v6 would be more than adequate for most.
In my case, I tried to even have ford put in the 3.31, but they said even though it was factory option, I would have to use a tuner to reprogram for the gear (this was downs ford, which is (or was) a huge FMS dealer and does a lot of aftermarket installs). I figured once I used a tuner, even for just gear ratio, ford might not believe me and thought I tuned the car if anything in engine/trans effed up.
When ford put huge rebates on the 14 GT's, it was too tempting not to trade up (helped that I got 14 V6 base for 19,2K from Koons and traded for 18 at a different dealer).
So need more info on what exactly you have and what your use and concerns (warrantee) are.