Swashbuckler lasts for about 5 seconds, same as Kill Clip. I ran a quick comparision using a Better Devils with Outlaw and Kill Clip, which gets 1360 on base damage and 2080 with kill clip. +5 stacks of Swashbuckler gives you a whopping 65% damage boost on a sidearm. The tradeoff is that Swash requires you to either get 5 kills or get a melee kill to get those 5 stacks. Swashbuckler has a higher dps at 5 stacks than Rampage at 3 stacks.
Surrounded in PvP is much harder to proc and to use successfully than kill clip for the same buff. TL DR for Warlocks and Hunters: Kill Clip is the best in most situations, but not all, for getting a major buff quickly and easily. – A PvP Guide to Damage Perk Selection for Every Weapon Archetype.