If your choice is Narrowband or NTL BB, I would take the NTL BB in a heartbeat. Generally the network performance is good (nationwide), there are some problems, other parts of the NTL infrastructure creak, Mail and News being the primary problems (especially when compared to the spam trapped Demon mail and their quality news servers - if you use them).
Of the three service levels for you I would think the 750/128k would be best, this would give adequate capacity up to the network for the positional data etc on flight sim. If you have no problems with that on dial up though, you could start with the 300k and be about break even on changeover cost.