I would go with Vitality + Dexterity.
I had about 5k hp in Nightmare and was dying constantly. By the time I got to Diablo I had upped it to about 13k and my playing was considerably less, well, dead. Vitality is extremely important to any class, despite whatever the primary attribute(s) may be. Once you get to the harder difficulties having health is a definite necessity.
A combination I've recently discovered that goes well with Dex + Life on Hit (assuming you already have decent hp) is taking the Sharpshooter perk and some assorted +crit chance/damage, resulting in very frequent crits and essentially constant healing.
Despite attack speed not having a cap (as far as I know) and life on hit being very useful, you don't output nearly enough damage to sustain the damage Hell/Inferno enemies output. It's better to be able to take a few hits and escape than get (nearly or straight up) one-shotted by normal mobs later on and slowly regain health. However if you play a lot of co-op this may be more successful, although you would have to make avoiding damage a priority, which would detract from your damage output.