I have a 700m for home use, and have had an opportunity to briefly use the x300 at work.
The X300 is definitely sleeker - much thinner and lighter. However, you lose the optical drive and the battery sticks -way- out the back. The screen is smaller and the resolution is pretty low. The keyboard is excellent.
The 700m is thicker, but otherwise about the same dimensionally when you account for the big battery on the x300. It also has the advantage of having the optical drive built in. The screen is brighter and the resolution higher - the wide screen aspect is also cool. The keyboard is more cramped.
Both machines are pretty sturdy, but I'd say the x300 is a bit sturdier. The X300 will be more portable if you don't need an optical drive - and less portable if you do. I -really- like the 700m, but probably would have went with the x300 if the price were the same simple due to the 'cool' factor.