The main differences are the number of rows of balls, whether they have wipers/seals, the height of the carriage above the rail and the loading they can take, whether the carriages are flanged or plain, and the profile of the rail (some profiles are better than others for sideways or twisting loads).

Since you have little sideways force (no moment due to cutting forces) a relatively lightweight rails such as HG15 or HG20 could be used. HGH15 can support 2tons static and 1ton dynamic (thats the max load for a specified wear in so many km of motion). MD sell a pair of HGH15CA bearings and 2 x 1500mm lengths of HG15 rail for £275+vat

Cheaper still would be the MG12 series, being only a double row of balls its more lightweight but still good for 1/3ton which is easily good enough for your needs... can't find an online price tho.

The other product to look at it the IGUS DryLin