As Matt has said, the Vertex 5" would be perfect (throw the swivel bit under the bench just in case you sell the vice at some time). A 6" one, I think, would be pushing it a bit.
The rule of thumb is to measure the table width and double it, that is the OAL of the vice it can comfortably handle. It just so happens that the jaw width on the K series vices are approx half the length of the vice. I think the Kurt style are slightly shorter, so you may be able to go up a size if you went for that style.
Again Matt is spot on with the info for the RT.
Things like these always tend to look too large when fitted to your machine, but you should always try to get the largest that will fit the table. If you have an unequal number of t-slots, try to mount the equipment to the centre one, attempting to keep the table balanced front to back.
I am lucky in that my machine has a fairly long table (36" x 6"), and have managed to squeeze both on at the same time, and still give me a reasonable length of cut in the vice. It saves a lot of time in the long run by not having to reset the vice whenever the RT is fitted.
Bogs