Hun IIRC, in your other thread you mentioned not liking the midwife that attended DD's birth. Are the NHS MW's on a caseload or is it pot luck who shows up? If it is pot luck, how would you feel about getting the same MW or one with a similar attitude? If you want a relationship with your MW, to be able to bond with her, and know that you will have that support during birth and the NHS is not caseload, then I would go IHBM, if you can get caseload, and therefore know exactly who will arrive when you are in labour, then go NHS.
Theoretically you get a named midwife who should show up when you're in labour. In reality they do not provide this. They can't, they're SO stretched. So it'd be pot luck, but TBH after my last birth i'm perfectly happy, whether i have an IM or NHSM, to call when i'm starting to push and just have the midwife for the last few moments of labour and first few hours afterwards anyway. If i'd done that last time (you never know "when" that is with #1!) my medwife, sorry midwife, would have had far less scope to change the course of things. I have great support and do better without strangers during labour anyway, so it's not a huge concern either way. I could certainly try to push for a named MW again though, since it's something they're supposed to provide.

Bx