I use both on mine (3" and 11"). I use the 11" for my corn that I have in 41 qt. tub and the 3" for my bps. The biggest things to keep in mind are the wattage per foot of the size of tape you're using, as it can vary. The wider tape (11") won't necessarily heat better or more efficiently than the 3" due to having a lower wattage per foot than what the 3" does, depending on what you buy; there are different wattages in the different sizes so keep an eye on that when you're ordering/building stuff.

Another thing to keep in mind is running different size tapes off the same thermostat. The 11" tape generally has a different wattage than the 3" and if you run them off the same thermostat, you'll get spikes or lower heating levels because of the resistance.

Bear with me here and I'll explain, lol: say the 3" tape has a rating of 10 watts per foot and the 11" has a rating of 4 watts per foot. The way electricity works, in simplistic terms, is that it takes the path of least resistance. With that in mind, the electrical current running to the 11" is going to heat it up faster than the 3" due to the lower wattage requiring less electricity to heat up the 11" tape to the temperature level you set. That'll cause spikes in your temps because the two different tape sizes will heat differently due to the difference in wattage.

I REALLY hope that makes sense and doesn't confuse you, as I'm trying to help, believe it or not, lol! Basically, don't run two or more different sizes of heat tape or different wattage ratings of heat tape off the same thermostat. I run all of my 3" tapes off one thermostat (all of the 3" has the same wattage per foot) and all of my 11" tape off of another thermostat to keep my temps where they should be.

Good luck and I hope that helps!