In your original post you don't specify if you're looking at a Kestrel with built in ballistics program. If you are, espec the Applied Ballistics version, then I'd recommend the BT option. I just "upgraded" my 4500/Atrag (no BT) to the Applied Ballistics with BT.
The ability to store data on a computer is nice, and the data cables are at least as expensive as the BT upgrade. Brian Litz and the team at AB seem to be committed to continual upgrades and an AB website, etc, to make maximum use of the connectivity between the Kestrel and a computer.