If don't yet have an uq or a tq and can only afford one, I'd go with the underquilt. This is especially true if you can use an old sleeping bag as a tq in the mean time like Mancat reccomended. I learned the hard way that you need insulation under a hammock just as much or even more than you need it above. I'm planning a DIY project to make a tq, but I've been rocking a HG winter Incubator uq and an old sleeping bag tq for a while now, and it works just fine.