Engineer and Elementalist
I’d pick your class based on how much you enjoy them in your favourite game-mode. Engineer (and Elementalist) both take some times to get used to, Engineer took me longer than Elementalist. If you enjoy Engineer go for it full-out; research builds, practice rotations, watch skilled players play the class etc. I can’t speak for sPvP since I kinda suck at it, but from reading the complaints people currently feel Engineers are in a better place than Elementalist. So it might just be skill/set-up related.
For WvW they both have their places. Engineer is relatively popular with healing bombs for some team-support and Grenades for some damage. Ofcourse they’re plenty more viable builds, just giving an example. They make good roamers aswell, condi-perplex, surely there’s a guide somewhere. (Note: Perplex might get changed the 15th, I don’t know.) Elementalist has 2 pretty decent roaming builds (I’m only familiar with D/D myself) and for zerging you’re basicly stuck to Staff, which means you won’t be juggling Elements as much (you go to Fire when offensive, Water whilst recovering from a push, Earth/Air for Control.) But a lot of people feel it isn’t rewarding enough, I personally love my Elementalist in WvW though.
In PvE Elementalists are likely the squishiest class you’re going to find. Low HP, low Armor, but they’re also one of the highest DPS-classes and provide good offensive support (so does an Engineer, but Elementalist also have their conjures so ye, they’re more ‘meta’ than Engineer) Where-as Engineer will have a bit of an easier time staying alive with similar damage. But I wouldn’t base your class-choice on your least favourite game-mode, even though you have to get through it for WvW.
It’s much harder to come up with a viable build for engineers that can compete damage-wise in PvE with most other classes due to their lack of a melee weapon and lack of reliable condition procs (bleed particularly, it’s the primary condition damage). So far, in all 8 classes, it took me the longest to min-max. Initially they’re easier to play though. It’s easy to learn and hard to be good at – since the number of viable builds is pretty much only two (grenade and bomb/rifle).
Elementalists are without a doubt the hardest to survive in at first because their passive defense is very, very low – but their active defense is also very, very high – actually, the HIGHEST of all the classes, outstripping the Guardian by a thin margin. In PvE fights against AI this makes Elementalists the most powerful class by far when you combine their might stacking capabilities, their insane active defense and their utility skills. However, against other players who attack with far more frequent high-damage autoattacks and much shorter telegraphs on active skills, Elementalists remain one of the weakest classes in PvP content due to this low passive defense issue.