IS that enough to argue since I am the one paying the money? DH and I need to be happy with our decision. I am willing to seek a 2nd opinion (go to another OB) if he doesn't budge, and am just praying that I can find one that will accept new patients..
You don't even have to argue on the basis that you're paying the money.
This is your body, your baby, and your birth. You simply say to your OB "Thanks for your advice, we have decided that we will be having our baby in week 39, can you book us in then".

END of discussion.

Without your consent he can't do it anyway, perhaps he could do with a gentle reminder of that.

Don't approach him like you're asking his permission, you don't need it and nor should he make you feel like you have to get it. If he sees that you feel that way, he will use that position of power your putting him in to call the shots. But the shot-calling needs to come from you! Your baby isn't sick, neither are you, there is no reason he should be telling you what is going to happen and disregarding your needs. Make no mistake, his preference for you to be booked at 38 weeks probably has alot more to do with his trying to juggle other clients than it does about you or your bub.

Stand up and tell him what will be happening with your birth. And the "small pelvis" thing is really worth investigating further as well. NO ONE, obstetrician included can tell if your pelvis is too small for your baby before you've laboured and this is really very rare, at any rate.

Good luck