A 0 degree would be way to hot in the summer IMO even with venting it. As Slo said, getting a 20 degree and adding to it in the winter and venting it in the summer might be a better option if you only want to buy one.
You're pretty tall so I can see why you wanted the longer hammock. Some members have been relaying their experiences with shorter hammocks being more comfortable for their given heights. A BB might feel just fine for you. I have certainly read some people the same height as you that have had no problems getting comfortable in a BB. Maybe someone with both an XLC and a BB can give you better input.
Typically resale prices are high enough that you would not lose a lot of money if you found you wanted to sell what you end up with and get something else.
Don't let life get in the way of living.