which ever you can get cheapest.. would be my vote ... and free is better ....

in theory.. they should be timed out to be up potted or go out into the garden.. before they need repotting.. so why would it matter??? .. if left too long in any pot.. winding roots will be a problem ... and the shape of those roots shouldnt matter ...

ive used 6z solo cups.. and reused a bazillion cell packs.. and i cant think there was any long term problem i could relate to the shape of either ...

and dont get me wrong.. i had plenty of problems.. lol.. but not pot shape ...

ken

ps: quick hint... wash out all your pots/tools/water can/etc with 10% bleach ... and sterilize your media before you start ... start clean.. and avoid many problems ...

https://tipnut.com/sterilize-soil/

Either/or. Historically, square is easier to find, and square is a better geometry to fill the (rectangular) flats efficiently and with a minimum of waste space and maximum amounts of soil and root space.

So I've always chosen square-ish (a little rectangular, I guess) when there's been a choice.

Since everything is usually fully root-bound by the time it goes into the ground in May, that always seemed sensible to me, but I doubt it really matters very much. :-)

Yep, doesn't matter. I use square for seedlings because the fit is better into rectangular trays, but for the bare roots I pot up I use larger round ones because that's what I have -- I saved a bunch of them over the years from when I bought plants, why buy new when you can get them free?