There are two basic types of racks:
Square Hole and
Round Hole (or
Dell's Rapid Rails (and similar rails sold by other companies) are designed to be installed in Square Hole racks, usually quickly and without the need for tools.
Dell's Versa Rails (and other Round Hole / Threaded Hole) rails are designed to be installed in round-hole racks.
Round Hole Rails can also be installed in square hole racks by using cage nuts, which snap into the square holes and allow you to screw the rails in place.
Bottom line as Mark Henderson said: Pick the rails that fit your rack.
If in doubt (buying equipment for a rack you've never seen), get round-hole rails and some cage nuts.
Dell also offers both Static and Sliding rails. Both rails allow you to slide the system in and out, however Dell's Sliding Rails (and cable management arm) allow you to extend the system fully while it is still connected to the rack infrastructure (power, network, etc) - provided of course your cables are long enough to allow this as well.
In practice I've not found a substantial difference in maintainability using the cable management arm / sliding rails (and the cable management arms tend to create heat build-up in densely packed racks by preventing the fans from efficiently ejecting heat out the back of the rack), so either style of rail works in this case, but I suggest not using the cable management arm. Your mileage may vary.