I have a 50 Marauder, 50 Guardian, 30 Shadow/Sage so I will do my best to answer.
The main difference between Marauder and Guardian is utility.
Sent : IMO is one of the best healer killers in the game for PvP. Their talents allow for one of the shortest interrupt cd's in the game as well as a move that reduces inc healing. They can also dish out a massive amount of single target damage, especially if being supported by a healer. They also have quite a few abilities which allow them to soak up damage, and stealth in combat if they wind up getting focused.
In PvE my Marauder (champ/columni gear) can pull boss aggro off of a tank in HM's if I go full burn.
Their main drawback is they are one of the only classes in the game that has no push/pull ability. This can make matches like Huttball extremely annoying if the enemy gets on one of the catwalks, you have no way to get them off.
In PvE they also fall prey to what every melee class in every game has issues with, and that's positioning. Any boss that knockbacks/aoe/swipes etc means downtime for your dps as you are not able to hit the boss unless you are in range. I've never had an issue with enrage timers, but you do need to move more than say a Trooper who can constantly dps even when moving.
Guardian: I run mine as a tank, but 1.2 made their Vig spec quite powerful as well. Their DPS is never as high or bursty as a Sent, but even with that as a full tank spec I still land hits for upwards of 4k with only one piece of Battlemaster gear so far. In PvP I have the utility of not one but two leaps, as well as force push for getting people away from objectives. Also guarding in PvP means 3 medals within a minute after the WZ starts.
Survivablity in PvP is meh at best, simply because of how the defensive stats work in PvP. With warding call, focused defense and saber ward I can basically become immortal for 10 which makes ball running in Huttball extremely fun.
If you want to DPS in PvE I would say go for Sent over Guardian. You wont have the burst/dps that most HM groups are looking for unless you work on your gear extensively before going in on the Guard, and the Sent can offer more overall to the group than the Guard can. If you want to PvP, it really is up to which play style you like better, lower dps for higher utility = Guard, higher dps for lower utility = Sent.
You ruled out Sage so I will skip it.
Shadow: To me Shadow is one of the best solo killers in the game. If they get the drop on you, it can be a hard time for any class to shake them off. You have essentially 4 interrupts if you use your skills correctly, one of which is a knockdown that can be used out of stealth if traited correctly. They have slightly less straight DPS output than say a Sent, but again have more utility/mobility because they have insta cast abilities that can be used at range/on the move.
Their main drawback is light armor, which can make them squishier than the other classes, especially if you did not go up the tanking tree.
Sorry for the long post, let me know if it helps.