Woah, back it all the way up, we also need to know....what amp you're getting or have, any electrical upgrades? Like the big 3? What size wire you are running, what kind of equipment do you have or planning on getting(such as speakers, HU even will help), etc.
Since you're asking to get 2 18's or 15's, I guess you want the max SPL you can get, do you want any SQ? I mean, with that much bass you're not going to even hear your music, are you planning on upgrading from stock speakers?
18s need a lot of space and I kinda doubt I have room for 2 in my Blazer. Like bl0ckd said, you're probably better off getting 2 12's. It's gonna take up a lot of trunk space. Also, if you could, list the budget you're looking to spend please.