depends. Personally I would rather have a keep, with a large amount of grass on all sides, than two 18x18s squashed next to each other.
If you're an absolute sucker for customization, go with the 18x18s, otherwise the keep holds many advantages - one less account to pay for, tried and tested "impressive" design, great fun to decorate (custom houses just aren't as fun to decorate as classics, cos you find yourself making a design to suit the decoration you wanted - classic houses forces you to be creative with your items, which is great fun, again imo)