if you want your slide nitrided after the cut, not too many places do that.
JagerWerks, and I think
Maple Leaf. if you're okay with bare metal on the cut surfaces, or cerakote, then many more slide machine shops are available.
I've used Jagerwerks,
Nameless Arms, both with good results, and maybe some others, can't remember shit. But nowadays, if I can buy a new slide with the configuration I want, the finish I want at a decent price, I prefer that option.
If you send JagerWerks your suppressor-height iron sights (e.g. Trijicon GL201-C-600649) and your optic, along with your barrel & slide, they'll mount and zero them in free-of-charge. That was nice. Mine were spot-on right-out-of-the-box (YMMV). On my JagerWerks-done slide, their nitrided finish is beautiful. It was a Lone Wolf bare stainless 10mm G20 slide. I also had them RMR-cut a LWD 10mm G20L slide. JagerWerks, Maple Leaf, and Nameless do the little RMR recoil lugs in their RMR cut. Some vendors don't, too much humbug.
take a look at
TR Enabling slides, although the G19 Gen3 RMR is out-of-stock at this time, hard to beat the $130 price.
I think it depends on whether the slide you want is commonly available e.g. G19 Gen3, or something less common. The G19 Gen 3 being a commodity nowadays, many vendors offer G19 slides with optic cuts at great prices.