I have a lvl 80 Engi (my only 80, actually, so I play it a lot in dunegons and stuff) and a Mesmer purely for PvP. I do PvP on the Engi as well, I tend to switch around a lot to keep things from getting stale.
First off, Engineer has a few builds that are very good, namely:
- Rifle/Grenade Berserker: a full-on damage build that has great AoE damage through Grenades and great single-target damage through Rifle, as well as access to handy conditions from Grenades (such as Poison and Chill). Negative side: Grenades are spammy, so expect some buttonmashing.
- Bomb Kit Tank: a build with high survivability, it uses Shield for CC and Bomb Kit for AoE damage as well as Blindness. The "tankiness" comes more CC and Blind than gear. Designed to control Capture Points in PvP, great build there. Again, bombs are spammy so a lot of button pushing involved.
- Flamethrower/Elixir Gun Support Hybrid: my current build for dungeons and general PvE. Nice damage through almost continuous Burning as well as great support due to Weakness application and a Healing field from Elixir Gun. Feels contributing in almost every situation, I'd recommend this build greatly. Also offers a bit more variety in playstyle.
- Greatsword - Any
- Scepter/Torch Confusion
Bottom line: create one of both and test it out in the Mists. If Grenades, Bomb Kit or Flamethrower don't appeal to you, don't pick Engi, cause Turrets and Gadgets are inferior to those three kits (they're not bad or unfun, but hardly as efficient as those three kits).