theres a few things that need to be taken into account here...
first thing: YOUR SKILL LEVEL! Any wheel can slide if you got the power/skill to do such a thing
The type of board: If you got a kick tail its much easier to pop those back wheels out for little slashes on mega soft wheels... which will still grip
Type of sliding: are you going balls out trying to do 720 pressure spins and massive surrender slides or are you just keen on doing standup slides on your longboard?
what sort of board are you going to slide on? different types of board allow sliding (stand up and drifting) to be easier or harder depending on their shape, be it drop thru, lowered or top mount and such...
So then to answer a few of those questions quickly.
Wheel shape and size: A smaller wheel will always be easier to slide. Something with a rounded edge will want to break out easier. 'slalom style' wheels with sharp edges (like fresh o-tangs) will be hard to slide out untill you break them in. It also depends on the duro how easy they slide out. You can slide the fuck out of a set of o-tangs and they will always grip, they may loose a bit of traction over time but they will still grip... the only time you will notice is if your trying to rail a hairpin corner at 60k's+ and want the fasted line, you will feel the wheels scrubbing...
Otherwise the wheels will not grip if they are shredding and worn to shit... thats a given.
the type of board: Ultimately, a topmount board will always be gripper. There is more pressure on top of the wheels to force them into the ground so they want to dig in and grip more. You can still slide out on top-mounts once you lift your pressure off but you will always get more grip.
A lowered board or drop-thru board will be easier to stand-up slide becasue the pressure your putting on the wheels while sliding will be more sideways then downwards. As you slide your pushing the wheels with an angled force rather than dragging them with the top of the board. You will find it easier to slide 'fresh' wheels with a lowered board.
But pretty much what it comes down to is the quality of wheel. If your buying a set of o-tangs, retros, seismics, sector 9s or any of those big name brands they are ALL designed to allow sliding, speed checking and drifting whill still keeping as much grip as possible... you will only start to loose grip after you've ridden them fuckers to the ground.
Moral of the story: Ride your shit till it dont work. Then buy more shit... then go skate more