I really don't think it matters, but if you really need a tiebreaker, the round shape has less surface area per volume, which theoretically would be better for holding temp.
I have rectangular 50qt with the valve at the center of the long side. Usually you see the valve on the end but not this one for some reason. The braid inside looks a little awkward like it doesn't know which side to rest on (hah). I've thought about making it all fancy with a tee so it has an arm going each direction, but I haven't had any sparge issues yet. First AG got 77% and second got 68%. Hit all temps dead on, so I write up the fluctuation in efficiency to the crush (I don't crush my own yet).
+1 to go big now, I plan to do 10 gallon batches someday but if you had asked me 2 months ago, I hadn't even thought about it yet.