I’d also go with 700c for the same reason that I ride 29" MTB wheels instead of 27.5".
Another plus of the 700c is that with a 25mm inner width, you can run anything from a road tire to an XC MTB tire depending on what fits in your frame. I’ve run 650x2.4" in the past and they felt slower than my 29x2.4" tires. 
There was a noticeable difference in rollover too.
Another thing to consider is BB height. If you ever want to run a smaller tire, it’s going to be harder to find something nice that’s smaller than 47mm in 650b and your BB height will start to get pretty low too.
@BillyWaldman to each their own. I’d have no issue running 45s in the conditions where I’d take a gravel bike. The original post mentions that their frame can only fit 700x46, so I think this is a valid point here.