I would not go cheap. I like using the cable-type thermometer for reading the temps at grate level. If you can get one of those and figure out a way to put it on the grate that works best. The lid thermo is notoriously inaccurate. Although, I would not sweat it, go ahead and work with the 55 degree diff this time around. The only problem with the thermo being on the inside is that you cannot read it from the outside, and if you open the lid to read temp, you lose heat.
"perhaps...but then again...maybe not..."
careful there son, those ribs are boiling hot...
(='.'=) Here there be bunnies...
Pacific Rim BBQ
Bob's Brew and Que