Well, you'll hear many different opinions on this, but I'll start with mine! I like a 10 gallon MLT, even though I've yet to make a 10 gallon batch. I was leaning towards a 5 gallon round MLT, but the others here on the forum talked me out of that and into the 10 gallon. The reason is actually pretty simple- I can make just about any 5 gallon beer batch I want. I can use 9 pounds of grain, or 18 pounds of grain without maxing out my cooler. I can add mashout water to my MLT with plenty of room. I think a 5 gallon cooler would be maxed out pretty easily. I never tried a rectangular cooler, so that might be something to consider, too.
I batch sparge, but am thinking about getting an HLT and flysparging. I like having the flexibility to do that if I want.