I've always thought m.xxxxxx.xxx was better than .mobi, even though its the same number of keystrokes on a qwerty keyboard. Mainly because it means you aren't pushing another TLD onto the public who aren't exactly the most TLD-remembering people on the planet.
m.xxxx.xxx allows an individual company to market its mobile-website whereas xxxx.mobi relies on .mobi being successful. If I was asked to choose between them for a business, currently, I'd say both however as in the long run, $10 or whatever for a mobi reg is a drop in the ocean if you're a business, especially a big one.