If your wife is like most wives, you should take her to a bike shop, and let her pick the bike she'll ride. You could always install some smoother tires on her mountain bike, just to get her out on the pavement. If she enjoys that, then go bike shopping for something more suited to the roads and trails. My wife loves her hybrid, and won't let me buy, or build, her another bike. She picked it out, though, which makes it "hers". The bike I bought for her hangs from the ceiling in the garage, and hasn't been ridden in years.