700c or 650b ????
Looking at 700c x 45c or 650b x 47c

Depends on the gravel size. Anything up to 20mm is 650b and anything over 700c.

A big Q : )

Ridden both of those combos a fair bit and tbh there’s not a lot in it at all. Comparing WTBs tyres the 650×47 does have a fair bit more volume despite only being 2mm wider in stated size. Rolling feel is very similar – OD gain of one seems to balance the added volume and cush of the other, once pressures are about right for each. You can hit roots etc harder with the 650×47 though, the wheels are perhaps a bit stronger/stiffer and you get more guard space with 650s, but overall I’d say it comes down to subtle differences.

If I was biased more towards off-road or a 50/50 mix I’d go 650. For a bike that gets used on road ~75% of the time and needs some off-road adaptability here and there, 700.
Oddly maybe having said that, I’d still go for 29″ every time for MTB so this is all about tyre volume within frame space/geometry that feels good on road.

More tyre choice at 700?

Yes
Am finding that there are not many 650b gravel tyres

I’ve got both wheel sizes for my Arkose and although it’s very early days (only had it two weeks) I think I prefer 650b 700c and as said above – there’s more choice.

It’s fun having a bike that you can swap the wheel sizes on in two minutes though.

700c

650b

Depends on the gravel size. Anything up to 20mm is 650b and anything over 700c.

Interesting, because for me I’d say the exact opposite.

Usual set up for easier (less mmtb like)gravel rides for me is 700X40 Nano’s and when the going gets tough I use 650b x 50 Spesh Renegades.

My choice of tyre is irelevant to the general concept that I will go to 650b wheelset for the toughest rides, those that really blur the lines between gravel and mtb rides. I’m always looking at new developments in rough stuff (sorry thats what I call gravel) tyres.WTB Resolute are likely to replace the Nano’s in the near future.
I do very little road riding on my gravel bike,so this may be different for the OP.

The 2017 Cannondale Slates came with 650b wheels and the review comments I saw seemed to complain that there is a lack of tyre choice available. Interestingly, Cannondale are not importing the Slates to the UK in 2018.

If there are few tyres available now, I would not risk it myself as I can’t see there being many tyres made available in the future as it seems more niche than most niches to me.

WTB Resolutes are available in 650b x 42c, standard fitting on the 2018 Slates…interesting they are not coming into the UK next year.

wind-bag

Resolutes are available in both wheel sizes. They look really good to me on paper and the few reviews I’ve seen. I’ve always been critical of the Slates limitation to 650b only but I think the new Resolutes will improve the bikes ability to deal with UK conditions(if you could get one obviously)

I have a pair on my Slate, plenty of clearance in the rear, they improve grip no end.

I really like my 650b wheels but there is definitely more choice of tyre for 700c, especially towards the muddy end of the spectrum so I plan on keeping both & using the 700c with more cx style tyres in the winter.