Both are good choices so there's not a wrong answer. I had bad luck early on with IR, but because I've heard they've gotten better and the price difference was substantial, I bought an IR drytop (not suit) when I needed to replace my drytop.
My complaint with the IR I'd bought in the past was that it felt great at first, but within a year was leaking a lot. This one seems to be holding up better, but not as good as Kokatat.
I'm going to need a new drysuit in the next year or so and plan on going Kokatat again for that. They seem to have better longevity and what's key to me is you can send it and get it pressure tested and patched. I'm not quite sure how that works with IR, but I'd find out the answer before you invest in a drysuit.
I put a quite bit of use on my drysuit in a season and currently have 7 seasons on it. I've sent it back a few times for patching. I sent it in last summer and it's still quite dry, although there's a bend in the zipper that I'm worried might fail at some point.
Now that said, the pricing doesn't make the choice as obvious to me as it used to be. If you send in your suit to Kokatat, you can end up spending upwards of $200. If you can get a sale on a Stohlquist or IR and purchase it for $600, then you're looking at spending 1/3 of the cost just for a maintenance repair. But if you're looking at $1200 Kokatat vs. $1000, then it seems like you're still better off w/ Kokatat.