My DS was 20 months the first time I took him to DL. We did almost all of the rides except for the Mountains, and had a truly magical time. He loved the characters, and was completely mezmorized by the Electric parade (which is now at DCA). Staying on-site, you will have rights to EE, with most of the Fantasyland rides open. I think its very worth going to DL with an 18 month old, even if you are going to WDW later. If you do go, use baby-swap to ride Indy; the only thing like it at WDW is Dinosaur, and Indy is so much more fun and exciting IMO.
I recommend GCH over DLH with your toddler if you do go to the parks. I've never stayed concierge because I usual only travel with one other person, and so the perks don't seem to be worth it. The pool at the DLH may be more fun for kids, but won't be a big deal for the toddler. On the other hand, the location will be a great perk, especially if you spend much time at all at DCA. GCH is a lovely hotel. DLH is still nice, but best suited for families with kisd.