Between both, which one is in your opinion better and more fun in all aspects of the game. Weapons to focus are GS and staff, aspects include but not limited to; PvE, sPvP, tPvP, ZvZ, sWvW, dungeons, Open world exploration, etc…
go
Obviously Mesmer Guardian > Guardian Mesmer.
Mesmer Guardians have a variety of builds, including Phantasmal Duelist inside your own Consecrations such as Hallowed Ground, and +Boon Duration on your clones to make them hit harder and last longer. There is also the Shouts on Clones build which makes Phantasmal Berzerker unstoppable because it now gets Stability and Protection. Since Shouts activate instantly, you combo very fast with very little chance for others to react.
Guardian Mesmers are very niche in that it only compensates for the lack of DPS of the Guardian only by using Time Warp, and also compensates for lack of mobility. Guardian has a very easy time applying Stability already using Stand Your Ground or Hallowed Ground, as well as removing tons of conditions, there is little point in using Mantras instead, specially since you dont have access to the trait line of your secondary profession.
Overall, neither of the combinations above are recommended compared to other combinations.
Mesmer Necros are much more powerful in terms of pure damage, having the ability to Shatter any minions, clones and phantasms. By going Mesmer Necro, you will practically always have your Shatter skills on cooldown though, so be careful when you actually use them.Necro Mesmers are also very viable, having heavy condition damage and other conditions. Having the ability to apply Fear then Daze or Stun immediately after can keep your opponent’s skillbar locked while condition damage is ticking down. You also get access to much better escape skills(since Necro have none) by taking Blink or Decoy.
Mesmer Engineer can shatter turrets, however turrets will not run towards your opponent like clones do, but you can then ensure more predictability because you always know exactly the locations where the turrets will shatter. A good trick is to stand still and cast all your turrets around you. Your opponent cannot get into the area then, because he will die in one hit if you shatter all those turrets with Mind Wrack. They are left with using range, which means you can easily take them down too with superior ranged DPS.
Mesmer Thief definitely gets more access to Stealth, which is very very good. Getting your clones and phantasm under stealth is strong, but very difficult to use. When you use Stealth on your clones, they do not stop auto-attacking, therefore your clones get revealed. To make a Stealthed clone reach your opponent unseen, you need to activate Mind Wrack right before Stealth, thus your clones stop auto attacking(because you activated a Shatter) and then become invisible. Positioning is also very important here because you might not get all your clones inside Blinding Powder if they are too far from each other. Also, Shadow’s Refuge does NOT work with clones, as they just keep auto-attacking, or walk out of the refuge and get Revealed.
Never go Thief Mesmer. There is very little point in doing so. Stealth is better than Null Field as a Thief, and 50% movement speed while Stealthed trait is as good as using a Portal or Blink anytime. The only saving grace of going Thief Mesmer is an extra Stealth skill as your Elite, there are way better options. Time Warp and Backstab also do not get along, because any smart opponent will get out of the Time Warp are before you are able to get behind them and actually land Backstab.If you insist on going along with Guardian, your secondary profession should be Ele. You can never go wrong with secondary Ele. Mist Form works with any profession, as well as Conjured Weapons. Guardians being very tanky can double their DPS for a short instant by using Fiery Greatsword, as well as continue to CC foes by using Lightning Hammer right after your own Guardian hammer skills are all on recharge. Ice Bow also makes a very decent substitute for the Guardian’s lack of viable range weapon.
I tried going Ele engineer once, I had up to a total of 41 skills(including elite)! It was quite weak compared to Guardian ele though, but I had a load of fun
But seriously though, have you tried out Necro Guardian? The added stability, aegis, protection and regen helps a lot. You might have less mobility, but you make up for that by being an unstoppable killing machine!