I was just looking at this the other day. I haven't actually touched the Pro, and so haven't physically compared the two, so a better answer will be from someone who does. But I have used the non-Pro version, and I did find the plastic ball-and-socket configuration less than smooth — which is what they're promoting with the new metal ball-and-socket. So, yeah, the metal construction is the main difference, but I don't think it's insignificant. It's not just a matter of durability, since it's a functional element that's constructed differently.
Digital Photography Review recently published a review of the Lensbaby Composer Pro and Sweet 35 optic, and in their conclusion, they note:
... it's the best Lensbaby yet, and substantially improved over the original Composer. In particular the focusing is very much improved over the somewhat loose feel of the older design, with a smooth, damped action that's reminiscent of 'proper' manual focus lenses. The ball and socket tilt joint has also been refined - again it's smoother in operation, which makes small, precise adjustments much easier. So if you think you're likely to use it a lot, we think the Pro is definitely worth the $50 premium over the Composer. Of course if you just want to experiment with whether you like the whole Lensbaby idea, the much-cheaper Muse probably makes more sense as an introduction to the system.
If you're interested in the Lensbaby system, the rest of the review is well worth reading.
↑ that's the "off-topic now" line :)
I think the name "Composer" is apt, by the way. It's appropriate for Lensbaby shots where you're working slowing and carefully. The Muse, on the other hand, is much more fluid and fun. It's also considerably cheaper.
↑ and that's the "way off-topic" line
My favorite, though, is the Lensbaby 2.0. The new optic swap system is cool, especially if you're interested in pinhole or zone plate, or if you get really into it and want to change between plastic and glass and so on. But the older models are considerably smaller, and the flexible tubing more flexible (and less squeaky, something I find a little bit disturbing with the Muse).