Haste works for cat druids like it does for anyone else, your regular attacks (white damage) happen faster. Most of a cat druid's DPS comes from special attacks (yellow damage), though, like Shred, Rip, and Mangle. About 1/3 of your damage is from Rip, 1/3 from Mangle and Shred, and about 1/3 from regular (white) damage.
If by 'Pvp epic cloak' you mean
Dory's Embrace, I'd take Dory's Embrace. Without knowing what your crit is already at, I can't tell you exactly how much better crit rating is than haste rating, just that it's generally better. 2/3s of our damage is affected by crit (white damage and Mangle/Shred), while only 1/3 of our damage (white damage) is affected by haste.