They will be different play styles. Typical of games like SWTOR, there will definitely be a class which theoretically does more damage, but it will be in Bioware's best interests to do as much to balance damage output across all DPS classes. This early in the game, expect many "flavor of the month" classes while Bioware gets a feel for balance.
Even after Bioware has a good handle on things, there are so many other factors to consider such as player skill, latency, and the individual encounter mechanics which are going to change who does the most damage for that fight.
TL;DR version - even if there was a simple "sniper/gunslinger does more" answer now, expect it to change, and expect it to depend on your skill.
i don't think you understand, gunslinger is the mirror class of sniper, thus they should be 1:1 but because a sniper uses a main hand weapon with a offhand melee i think it will pull ahead. then again the math may make them equal.