The tilt up tops are the cheapest but can be a pain in the neck when the cover gets covered with snow or ice. The clam shells are a little more but are much better, except you can't stand fully upright inside them or raise a sled hood all the way. The full height with door ramps are the most expensive, but are the best. You just have to keep the hinge area cleared out so it doesn't ice up. It is really nice to be able to stand up inside, and open the sled hoods.
Drive on - drive off is real nice unless you have reverse on all of your sleds.