Which modern, four-cylinder pony car: Mustang or Camaro?

Neither? The Mustang has the clear numbers advantage and likely in the aftermarket support based off the Focus RS. However, if I'm getting into a Pony car, it has to have a V8. Used GT350s are under 50K now and used 2SS 1LE Camaros are even cheaper. More money than a new 4 banger, but worth it. 

Daily driver:  Mustang.  I can see out of it better.

Track day car:  Camaro 1LE (although hasn't the 1LE been discontinued on all but the V8 model?)

According the journos, I'm an Alpha chassis guy.

Frisbee twins.

Mustang.  I don't care about taking a car to the track and the Mustang seems like the better all-around car.  Plus I like looking at it much more than the Camaro.

both these new cars are WAY to heavy but I think that eco boost 4 banger would be a fantastic swap into a 65-66 Mustang.

I'm a diehard GM guy but that camaro is ooglay.

In reply to 67LS1 :

I second this motion. I bet you could mount it pretty far back in the engine bay.

I also wouldn't be opposed to putting one in a Mustang SVO.

In reply to Colin Wood :

For all the effort and expense of putting a modern engine in an early Mustang, I'd rather have an LS3 and Coyote than a 4 cylinder Ecoboost. The mild increase in weight (and huge drop over an iron block 289) is well worth the noise and being NA. It would certainly be unique though.

I bought a turbo 4 Camaro for autox and daily driving. It's more than capable of keeping up with its V8 brethren at autox and in stop-and-go traffic. 

When shopping I much preferred it over the Miata and 86 twins for daily livability. The 370 is so old at this point it wasn't a real option, and I've known enough people to hate their new WRX that I wasn't interested.