Jeff,
Both the 18 and the 22 experience slight differences in temps between the 2 grates. It can go either way on each cooker, depending on your method of cooking. Usually for people that run water in the pan, the top rack is warmer. Usually for people that run an empty pan or clay saucer, the bottom rack is warmer. Not always the case, but in my experience it’s been that way.
Now my 22” has 3 racks, and I also do not measure temps at the grates… so I am not much help there! But I do have both the 18 and 22. If I only had to have 1, it would be the 22. Having more room on each grate is much easier to work with.
Not sure what kind of ribs you are cooking, but here are some comparisons for how much meat I have put on the grates.
18”
2x STL Spares or BB laying flat per grate
4x STL spares or BB in a rib rack per grate
1 untrimmed spares laying flat per grate
2x large butts (9+ lbs) per grate, they are going to touch a bit
~13x chicken thighs
22”
3x STL Spares or BB laying flat per grate
5-6x STL spares or BB in a rib rack per grate
2 untrimmed spares laying flat per grate
4x large butts (9+ lbs) per grate, they are going to touch a bit
~40-50 ABTs per grate
Either way, you will be happy. Hope this helps.