I had the same question... and wound up buying both. I guess it depends on the temps.
Last night's 1.2°F low in the 0°F Incubator was true to rating (just like the 20°F 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.