which is best for PvP? and which is best for PvE?

Depends on a lot of things, in general I would break them down as subt = pvp, combat = pve, and assassination = both.

But weapons, play style, etc can all change this.

So sub is best IYO for PVP?

Subt includes several talents help in pvp. Like prep for instance which resets your cooldowns. But by no means is a heavy subt build the end all be all in pvp. For instance I do very well in pvp with a heavy assassination build and others use combat builds.

A lot of it comes to play style. What weapons are you using or planning on using and then I can show you some popular builds.

im using the Fang of Crystal Spider in MH and Venomspitter in OH

For a dagger mh but no dagger off I would recommend either 31/8/12 or 21/8/22 for pvp.

31/8/12 will give you more burst and overall damage, but 21/8/22 will give you more control.

You could also pick up another dagger and go all the way to Mutilate.

also, avoid fancy daggers that proc damage or poison. stick with good solid +agi daggers. the agi adds attack power, dodge rating, and armor. in the long run you will do more damage and save more health with a +agi dagger than a proc damage dagger. also, for assassination look for a slow dagger for MH and a fast one for OH. oh and for pvp, crippling poison is a must!

im lvl 28 and I'm thinking about going from assas to subt. I love pvp and it will complement my play style. But I also enjoy 5-mans for leveling and PvE. I know I wont top the dps charts with subt, at least not easily, but can I still contribute enough to be desirable to a group(lvl 28-50)?

I've respeced 3 times and it all depends on play style. Combat Swords/maces with sinister strike.... any dagger build shouldnt use SS too much. I respeced my hunter once at like lvl 15. But the rogue needs equipment, talent, and skill rotation to be effective. Other classes are innately good at what they do, but rogues dont seem this way.

Many people will say unequivocally that one build is the best. The good rogues i've seen just know how to play their type of rogue. However, they usually say that combat is the easiest for lvling and can do well in the dps charts... its just boring and not "rogue."