Are you going to fly sparge or batch sparge?
If you go with a cylindrical cooler, you will have a higher grain bed depth to surface area ratio which can lead to stuck mashes more often. That can be controlled by your grind or the addition of rice hulls. I think it worked better for fly sparge when I did that early on before switching over to batch sparging.
The rectangular mash tuns have a smaller grain bed depth and it is easier to get channeling of your sparge water if you fly sparge. There are also corners that can be stagnant and not drained well if you have a poor grain filter set up. If you batch sparge this is not an issue at all.
I do mostly BIAB, but do have a cylindrical cooler mash tun that I use a couple of times a year. If I were to start over again, I would still get the same cooler, but would get a grain bag to fit it and use that instead of a braid filter on the bottom. That greatly reduces any issues with stuck sparges and makes them easier to deal with (just lift the bag).
