if you're not going to be taking really hard turns, like auto-xing, the +38 will give you the "poke" look.
you'll need to streach the tires a bit, which is not a bad thing in my books.
the +48 will still be kinda aggressive looking being that they are 8.5" wide.
my rims are 8.0" wide and the offset is kinda close to +48, so you can use that as a guage.
that's why i'm a huge fan of custom built to my specs rims.
My current wheels are 17x8 +48 so you'll be a 1/4 of an inch wider on each side, which is like 6mm out. That is pretty flush with a 235/40/18. Although, if you don't mind a stretch then go for the lower offset. I would think that with that type of wheel, it would look more concave with the lower offset.