Thanks for that. Following the links you provided, it's clear that the AF in the 7100 is improved over what's inside the 300s.
I'm really after a 24mp camera. I'm one of those who crop in very hard, so more mp= better as far as I'm concerned. I can use them all!
My DX camera is permanently coupled to a Sigma 50-500 OS, and as you know, it's not the brightest. I always use it a f8. I've 'speed taped' the wheel so it never changes! The challenge for the AF comes from the speed of the subjects, but in good light, the 300s does very well. It struggles with fast approaching targets sometimes, but that's no surprise really. I think I'd gain some extra AF capability in less than ideal light, with a 7100, but it's not going to be a huge advantage.
As far as AF points go, I never realised that some were different to others, but after experimenting with 51 points and various other modes, I settled on single point AF, AF-C and the AF-ON button method. This has given the best results so far, and of course- as long as you can keep that central AF point on the target- it seems to minimise the risk of the camera grabbing focus elsewhere.
I agree with what's been written elsewhere about the 7100 weather sealing etc. I'm sure that camera would do me fine- and if there was no prospect of anything better, I'd probably get one.
There's just that nagging doubt about the D400 being about to appear!
I think I'll use my 300s this year, and by the Winter, it might be clearer whether or not a 400 will be a-coming, or not.