Here's a little bit of information about the pros and cons of some of the plastic materials available from Shapeways. While they offer parts printed wide variety of materials, only a few of these are really usable for making models and props.
White Strong and Flexible - this is a laser sintered (fused) nylon material. Very strong and very durable. It's also Shapeways cheapest material since it requires the least amount of handling and they can bulk load a lot of parts in a single build volume. The term 'flexible' really only applies to thin section parts, in thicker sections it's quite rigid. The build resolution of this machine is 100 microns which is a little thicker than a sheet of paper. this layering is most visible on curved surfaces. Because these parts are made from fusing powdered plastic together the resulting surface has a slightly sandy granular texture, kind of like a sugar cube. You can finish the surface smooth using spray primer and wet sanding but it's a PITA and really not suitable for small finely details parts. But this can be a good material for props. Shapeways offers a smoothing post process for this material in which they put the parts in a vibratory polisher with ceramic polishing media. This will smooth the surface but also round over sharp edges a bit. The smoothing process doesn't reach into inside corders. The colored Strong and Flexible plastics are the same material as the white, just dyed after printing.
Acrylic Plastic - Parts are made from a liquid acrylic plastic polymer which is polymerized (turned from a liquid to a solid) using either a laser or ultra violet light. I believe the resolution of these parts is 100 microns, so stepping will be visible on curved and angled surfaces and it's not really appropriate for super fine detail. The material they call "transparent" is really translucent, not clear.
Frosted Detail Plastic - Shapeways offers two versions of this material, Frosted Ultra Detail and Frosted Extreme Detail. Both can yield really beautiful model parts. The resolution of the Ultra is somewhere between 25 and 50 microns which is very fine, but the build layers can still be visible on curved and angled surfaces. The resolution of the Extreme plastic is 16 microns, which is amazingly fine and any visible build layers will likely be filled in when painting. These parts are made by depositing a liquid resin layer ba layer which is then solidified by UV light.
As I mentioned in an earlier post, UV cured resins are not 100 percent stable and can warp, distort, discolor and get brittle with age. For small detail parts this is unlikely to be a problem. For larger parts, parts with thicker sections and structural parts, possibly more so. Keep this this in mind. The best long term solution is to use the 3D printed part as a patterns to be molded and cast in urethane resin.
Shapeways also offers parts printed in Castable Wax. This is the finest detail (and most expensive) printing material they offer. Intended for jewelry parts the surface finish is exquisite! Detail parts can be printed in wax and then molded in silicone and cast in resin. But the wax is fragile and for model parts, probably not really any better than the Extreme material discussed above.
Now if you want the ultimate in 3D printed longevity Shapeways offers parts in Silver, Gold and Platinum!
Hope this information helps.