Anything below 40 degrees in a hammock really requires a substantial under-quilt or you'll freeze your ass off. I've done a hybrid system. I have a solo (Tarptent) tent that's 28oz. I've also included a 8oz hammock system, and with my sil-nylon poncho/tarp, I can choose the best system for the temp/weather conditions.
The issue with hammocks (at least for me) in colder temps is that they be come excessively bulky and heavier than a tent/ground system. That said, anything over 55-60 degrees, I much prefer a hammock as I just sleep better. Additionally, a hammock will allow you to set up on very irregular terrain that is problematic even for a small footprint solo-tent (or even just a ground tarp). As most have said if the temps aren't too cold, but the ground conditions are soaking wet or swampy, that hammock is a godsend. Another consideration is that if you plan to travel at night and sleep during the day, the hammock may be a better option (if the temps are warmer) if you sleep better in a hammock. Don't underestimate your sleep or necessary rest. Most can go about 24-48 hours without sleep, but after that, mental errors turn into bad decisions, mistakes, and often mechanical injuries.
ROCK6