Yeah its tricky, there is no way to know how your ISPs points of presence hook up within the network.
As I have both Zen on TTB and Plusnet on BTW, I honestly can't tell the difference in real world use, though there is about 2.5ms first hop difference in favour of Plusnet.
I think the biggest difference is the cost if you later want to move to an ISP that is only BTW and you're on TTB? (or is that to do with the phone service, I don't entirely understand how the services work these days)