I'd say get an ATB to start. I've had ATBs and GTPs, and ATBs can handle more lenient conditions. Plus with a boa you don't ever have to worry about incubating eggs if you breed them.
If you were looking to spend more money, then I'd say a melanistic/calico GTP or a basin Emerald. I think the higher-end GTPs and ETBs are nicer, but for a lesser priced animal nothing beats the variability of ATBs. GTPs are usually more handleable though and I think make better displays than ATBs.
Here's some pics to help with the decision. Keep us up to date and post some photos when you decide and pick one up!
Baby Biak GTP
Same GTP at 2 years old
An Aru x Hormonal Blue Cross
Some ATB variability (all different individuals)
And if you like the dark GTPs, here's a couple possible het leopard ATBs