Arsenal or Pyrotech

I just wanted to know which is best for PVE, and soloing? Also what should I avoid putting points into in either tree?

Do whatever.

You can't say for sure what you'll like playing unless you try both branches. Stick to one til you get to the end then see what you're missing on the other side.

Very common question though http://www.swtor.com/community/showthread.php?t=535395

A lot of people like Arsenal and it's true in ideal conditions it can do more damage.

Some people like Pyro and on the flip side, it's easier to move while doing damage.

Pyro and Arsenal mercs can complete all solo, flashpoints and operations in the game. Just depends on your style.

Bodyguard is a solid healer too on another note.

Oh and your team of course, if your guild leader is fixed on the idea of one spec for the mercs it brings you're kinda screwed when it comes to choice. More enlightened leaders will take successful players of any spec.

*By "more" and "less" between the two specs I mean about 150-200dps vs one target in ideal no movement spamming which may or may not be how a fight goes.

I may revise that when I finally get a moddable offhand. They only became available in 1.4 for me which is a massive stat boost if I jump from a locked BH to an unlocked DG offhand.

This is the first class forum I've come to that didn't have any guides to help you out with stuff like which tree is good for what. So thank for the help I appreciate it.

Changed my mind actually on the dps difference, just parsed higher than I thought on a 10m run.

In L63 gear I'm certain a pyro spec can parse 2kdps+

Not solo, it requires an armour debuff from a jugg, sniper or an arsenal spec merc. Which is readily available in a raid environment.

That increases the 75% kinetic/energy damage a pyro does in an ideal rotation (taking into account penetration on certain skills already)

Also factor in the 30% increase on DoTs as a target goes below 30% health.

All in all it's about a 7-8% increase in what a pyro will parse solo on a dummy when assuming its armour stops 30-35% kinetic/energy damage.

My recommendation would be to decide whether the gender of your mercenary is one that you would enjoy watching bend over constantly. If it is, then Arsenal. If not, then Pyrotech.

Pyro is considered the better dps spec

So what in the Pyro tree should I not bother putting any points into?

Arsenal is better by a long shot.

This is my spec:

http://www.torhead.com/skill-calc#3000cZMckZfhbbdGhrs.2

Cleared TFB HM with my guild using that. And everything before that.

Well, its really what you like. If you like to move around, run, and blast people on the go, Pyrotech is your choice. If you use pyrotech, obviously use combustible gas cylinder. The key with pyrotech is that it uses alot of damage bonuses and over time damage. Incendiary missile doesnt do a ton of damage up front, but it has a great amount of periodic damage. It also deals with increasing the strength and useability and decreasing the cooldown of railshot with abilities like superheated rail.

Arsenal is more about blowing up in someones face with missiles and explosions, but you are much more of a sitting duck. Here, your primary ability will be tracer missile, and you will use high velocity gas cylinder. This class deals with lowering the armor of foes with tracer missile, then blasting away with high hitting attacks like unload, heatseeking missile, and railshot, and increasing the powers of your pushback abilities like jet boost