If you want lettuce that lasts into the summer, I'd suggest Jericho, a green romaine lettuce, and Merlot. Merlot is a large leaf lettuce that is crisp and sweet, and is stunningly beauiful. It's a deep burgundy red without a touch of green in the plant at all. The two together in salads are spectacular.

Merlot lettuce is pictured in the link below, apparently in a flower border. The flowers above it are red coreopsis. You can see the deep burgundy-red color that's closest to what it actually is, though it's much more vivid than can be depicted.

(You might be able to tell I'm quite enamored with Merlot lettuce!)