I'd definitely go with the shuttle. Why not? Only $7 each way when I was there in November and it leaves pretty often. You might have to stop off at a few hotels on the way so they can drop off other passengers first, but so what? That only takes a minute or two and you're on your way again.
One shuttle driver told me something about Vegas cabbies which makes a lot of sense. When they're driving you to the airport, they want to get there quickly so they can pick up their next passenger as fast as possible. But when they're driving you from the airport to your hotel, they might take the scenic route to jack up their fares...and of course, tourist passengers don't know they're being taken for a ride (literally and figuratively). I try to avoid Vegas cabs, but if I have no choice, I always ask up front how much it'll cost to get to where I need to go...