IME, wheat beers and high gravity brews (SG>1.080) tend to experience lautering difficulties initially when using a 10 gallon round Rubbermaid cooler. It is fine for the 100% barley and sub-SG 1.080 (non-wheat) beers, but the mash tends to get a little too thick as you run out of "container space". This really limits
Example: I just did a weizenbock (55%+ wheat, 33 lbs grain total, calc SG 1.091) today in the 10 gallon round rubbermaid cooler. Had a pound of rice hulls as insurance, as well. Before I even added water, the dry grist was filling up the cooler to the 8.5+ gallon mark.
There wasn't much extra room to add water if I had undershot my strike temps (which, thanks to beersmith, I didn't). But it was just impossible to stir all the way down to the bottom. I mashed for 70+ minutes. And it still was a long lauter--2.5 hours). I was kicking myself for not switching out to the rectangular coleman cooler for the mash tun.
Bottom line: using a rectangular mash tun keeps things simple and easy regardless of grist and gravity. It avoids grain bed compaction (as width exceeds height), thereby improving lautering. It also allows extra "container space" which makes hot or cold water addition (mash temp adjusting) and stirring much easier.
20 gallon SS Kettle w/ tri-clad bottom is the way to go--buy weldless thermometer and SS spigot and install them using a step drill bit. Use FermCap-S to prevent boilovers.