I think they are both EF-S lenses right? So for a APS-C sensor camera. Meaning they are roughly 21-120 and 15-28 in 35mm equiv.
I know someone with the 10-18 and it was a cheap lens. Not bad for the money. Think my mate paid £159 for it after a pricing mistake at a store.
While they overlap, they are definately ultra wide vs wide to mild tele zoom so have different uses. The 10-18 like any ultra wide takes some learning to get the best from it.
If you are thinking 10-18 as a way to get more in, then maybe think a bit longer as the ultra wides make everything smaller - that sounds an obvious thing to say but dont underestimate the effect.
The 15-85 is a general zoom lens that has focal length coverage for most shooting scenarios where as the 10-18 is a bit more specialised for landscapes and architecture.
To be honest, personally I think you would be better skipping it for now because I think thisnis one of those situations where if you are in doubt whether you need it, then likely you dont.
Just my opinion mind you. I think a nice prime like the 50mm f1.8 or the 40mm stm would be better but I dont know where you are on your photo journey and what you want to shoot. Hope my rambling helps