I had the same question... and wound up buying both. I guess it depends on the temps.

Last night's 1.2F low in the 0F Incubator was true to rating (just like the 20F Phoenix) but at that temp, the full length was quite the treat. I didn't even deploy the tarp - nothing got cold but my nose.

Some could argue the full length is a luxury - but that's part of the reason I made the jump to hammocks in the first place.

I'm sure it has been said, but you'll probably be very happy with your choice either way.