I got one for my AE86 off Hamilton Car covers - tailored, waterproof, all weather cover and fits nicley with under car straps. Well impressed the car stays bone dry even when its been hammering it down and even pooled water dosnt get through, but its also beathable so it will allow any moisture that comes up off the ground which can get trapped under the cover to escape. Cost 150 but well worth it IMHO. Also as its a good fit it dosnt flap about in the wind which can cause the cover to wreck the paint or just fall apart.
A tarps just going to ruin the paint and also trap moisture under it against the paint work - would be better to not bother (that is unless your cars not watertight in which case its a choice you have to make). Keeping water out is 1 thing, letting moisture out is equally as important as you find alot of mositure will come up from the ground and get trapped under the cover.