This may stir up some passion but I am wondering which is perferable-the round cooler or the rectangular one.
Bill
If you were going to fly sparge a round cooler would be preferable. But I saw your post that you picked up a 70 qt Coleman Xtreme. That will work great for batch sparging once you get it converted.
I like a round mash tun. There aren’t corners where mash can hide from being stirred up. But in the end that may not make that much of a difference if you are just diligent about stirring the mash.
Kai
The important thing is that it’s blue 
If you want to have false bottom you would go with round vessel (cooler).
I did not see any square / rectangle false bottoms.
With rectangle coolers people use (made) manifolds or use ss braids.
So it goes down how do you want to lauter grain.
I find that you get more cooler for your money with a rectangular. Also, the larger opening of the rectangular makes adding water and grain and stirring much easier. Kai’s worry of grain in the corners not getting stirred has never been an issue for me. A rectangular works equally well for either fly or batch sparging if you desing your lautering system correctly for the type of sparging you want to do.
I don’t think it should make a significant difference as long as the time /temp factor has been acheived. I use a rectangular cooler only because it was the size I was looking for at the time. Round coolers should work just as well as rectangular IMHO.
What makes you say that? I wouldn’t think that a round vs rectangle cooler would matter for fly sparging.
I was always under the impression that a round cooler made for better lautering when fly sparging however I see from the comments above that this isn’t necessarily the case, so I stand corrected.
I do have the option of picking up the 78qt Coleman extreme-question is this: Is this too large a size if I wanted to so a 5 gallon batch?
Nope, not in my experience.
Bigger is almost always better. So unless you’re cramped on storage space, get it.
There’s a certain point where that breaks down, but don’t sweat it.
All of my 5-gallon batches have had plenty of extra room in my 36-quart Coleman Xtreme.
When or if you start messin around with 25 lbs of grain say for a barleywine or other high gravity bill,
the 36 quart will be a disappointment…IMO
I want to try a coleman for fermenting in, I think it will work well for harvesting yeasties. 
When I start messin around with 25 lbs of grain just shoot me in the head :) The biggest I’ve done (and probably the biggest I’ll do) was a 1.075 IPA and the 13 lbs of grain for it plus water fit just fine in the 36’er. Most of what I do is in the .40s and .50s so the 36 quart is actually better for those as there’s still some decent grain bed depth that wouldn’t be there in one of those mondo size coolers. I guess it all really depends on what kinds of brew you do.
Well, I have the rectangular BLUE coleman extreme. I like the divot in the bottom
for the drain. It helps get the last nth of wert out. It allows me to make Either
5 or 10 gallon batches. AND I could always use it for the high gravity 25# of grain
stuff as well. It just allows a lot of flexibility for multiple varieties. I looked at the
circular styles as well, and the 10 gallon cylindrical gott orange was kind of spendy
compared to the blue coleman extreme. I believe without doing the math,
volumetrics are greater in the blue. As for the corners and dough in,
I use a large wire whip and have absolutely no problems with dough balls.
I use a large metal spoon and likewise no doughballs.
I currently use a rectangular cooler and it’s easier to dough-in with my mash paddle as I can break up the dough balls against the flat wall.