For me...I would take the V8. In any form...5.0 or 5.2-5.8 (whatever is in the next gen GT350's and GT500 replacement).
The 2.3 should be a promising platform, though. I would put good money on the aftermarket BOOMING for that engine...much more than the 3.7. You have to remember, many big companies who specialize in 4 cylinder turbo engines will expand to the Mustang. Much like they have done for the Focus/Fiesta ST.
Also, for what it's worth...the new Focus RS is supposed to have the 2.3 from the Mustang. The potential will be screaming out of these engines.
With all that said, though...I'd rather have a V8.