i guess we need more info to give you a real solution. what kind of bikes? what kind of terrain will you be driving to ride said bikes? and whats your budget for a rack?
for myself, i dont like carrying my bikes on top or in front of our vehicles cause i dont like the bug splatter. however, if your only doing short, close trips, those options arent bad. i use our roof mount a lot (on our civic), but rarely get up to highway speeds with it.
being in back has a different set of issues. offroad, the bikes get a layer of moondust. not a terrible thing and better than bug splatter, but either way you get something dirty on your bike. if its all pavement, on back is the way to go hands down.
rack style is going to also be a personal preference. for our civic i prefer the keep our bike in trays where front wheel stays on and rack grips down tube and has ratchet straps through wheels so bikes are ready to use upon getting to destination. ive used this style since around 91 with zero issues. roof of van isnt an option for me, but ive seen many setups on peoples vans where it looked handy on the back.
right now, i am using a 75 dollar craigslist rack made by hollywood that holds 4 bikes hanging on the van. depending on your bike, the hang style carriers can be hit or miss as well as so many bikes have completely different shock configurations. so that style will depend on your bikes designs. i like it cause it gets the bikes up off the ground so they dont drag on steep departures. since its only a 75 dollar rack, it doesnt hurt my feelers when i drag it on the ground.