As long as your grain bed is of good depth, it's not going to make a whole lot of difference round vs rectangular. Some prefer a 10 gallon round cooler because you know you'll always have a good filtering layer of grain and the bed won't be disturbed too easily regardless if you're making a run-of-the-mill ale or a big beer.
If you're making really big beers (especially in 10 gal + batches) or larger batches, rectangular coolers offer more and larger options.
I personally prefer rectangular MLTs, but just because that was what I started with and got used to the concept! Either way, don't worry about it too much, it's just an MLT!