The heat mat is great, you just need a thermostat. Hydrofarm from Amazon is the cheapest. . .I think $29 right now. Any heat lamp will be higher wattage than a UTH, I think they're usually 10 or 15 watts, can't beat that. So if energy efficiency is a concern the UTH is best.

You don't need to hurry with the new tank---it's not an emergency, but I do think you'll want to upgrade. I have a 700 gram BP in a 20g long, and there's just no room for anything. 2 hides and a water dish? Haha, no way that's going to fit. I have the smallest water dish possible and it still doesn't really fit, plus he tips it over all the time. Can't imagine a big girl like that in a 20g long. You can find leaker tanks cheap from craigslist or other classified ads. No reason to pay for a new water-tight tank if you don't need to.