Ive used both... but I really like my 10 gallon Rubbermaid round cooler with a false bottom.
Pros: works well with a perforated false bottom, which eliminates channeling (your sparge water taking the least path of resistance and not rinsing all the grain during runoff).
Cons: sometimes the runoff can be so good (if you crank it wide open) that a vacuum is created under the false bottom and can cause a stuck sparge/recirculation.
In my honest opinion I wouldn't even consider a square or rectangle MLT for anything but batch sparging, which works quite well being that you are draining the whole tun each time during batch sparging. All you need then is a good stainless screen or toilet braid to keep the grains out. But for fly sparging I would whole heartedly recomend a round MLT with a false bottom, then you never have to worry about bad efficiency because of channeling in the grain bed.
I got my cooler from Home Depot for $40, my false bottom from Midwest Supplies for $32, the SS Kewler kit from the same place for $18, and the bulkhead from Lowe's for $7... the whole thing was less than $100 and works beautifully.
my 2 cents anyway...
Good luck!