Should I get rackmounted or tower servers for in-house use

When you rack mount the servers you end up creating what are called hot and cold isles. The hot isle is the where all the servers vent their hot air into, and the cold isles are where you pump the chilled air from the AC into. In a server room it isn't about how many fans you can put inside the machine, it is about how much cold air can you get to the machine. Rack mount servers are configured to allow for the maximum amount of cold air to move through the machine as quickly as possible.

Rack mounted servers also make it much easier to get into the machines as you just slide the machine out on the rails do what ever needs to be done, then slide it back into place. When done correctly there is no need to unplug a single cable when doing this.

You mentioned space, but I'll talk a little about that. When towers they come in all shapes and sizes. If you start putting towers in a rack, you can probably fit 3-6 machines in a rack, depending on tall they all, and if you can put two on a shelf and have them fit. With rack mounted servers you can easily get 42 machines in a rack (assuming 1U servers and a 42U rack). That is a much more efficient use of space, and space costs money.

There are two basic types of racks: Square Hole and Round Hole (or Threaded Hole).

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.