In my guild who rips agro/has second agro is decided for the most part by who doesn't receive guard. We have had all 3 ranged classes rip agro at the strangest times, most noticeably during Karagga for obvious reasons and again, all 3 classes have done it. Also note I write this from a purely 16 man perspective.
As for Specs, our best Sorc runs a 0/25/16 spec. As I understand it frequent use of Chain Lightning with the 30% damage procs and the 3% global bonus beats Deathmark quite handily.
Now since the OP asked for a comparison between the 3 classes here is what I have experienced having played both Sorc and Sniper in a raid environment and speaking a great deal with DPS Mercs.
Sorc: Best sustained AoE, Single target DPS beats most classes easily. Comes with a powerful built in heal, Static Barrier and can cleanse Mental and Force effects (rare/non-existent in current content). Can interrupt. Force Speed makes dodging mechanics easier.
Overall brings the most frequently and generally useful utility to the table. Can spam a strong heal in times of need. Static Barrier is very useful. Interrupts are critical. Powerful sustained AoE helps in a few encounters.
Sniper: Competes for best single target DPS. Has a powerful but high cooldown AoE. Most specs come with built in survival mechanics to reduce damage taken as well as some defensive cooldowns. Can spec into a powerful accuracy debuff (Flash Powder 20% 8 sec) to serve as an additional defensive cooldown for the tank. Ballistic Shield 20% raidwide damage prevention is useful in many situations. Has an armor debuff in Shatter Shot which does stack with other armor debuffs. Can interrupt.
Overall a survivable if well played high DPS class with frequently useful (interrupt) and situational but powerful (Shield and Flash Powder) utility. Also increases overall raid DPS by bringing an Armor Debuff, the effective DPS of a Sniper exceeds their own personal DPS.
Merc: Competes for best single target DPS. Heavy armor and powerful defensive cooldowns provide excellent survivability. Healing capable though not as effective unspecced as Sorc. Can cleanse Tech and Physical effects which are the two most common debuff/dot types in current content. As Arsenal Spec as an armor debuff that stacks with other armor debuffs.
Overall highest survivability of the 3 ranged DPS classes. Armor debuff is overall raid DPS increase thus effective DPS is greater than personal DPS. Heal and Cleanse are useful at the right times. The lack of Interrupt is a tremendous downside, many fights in current content require interrupt.