[Damage]
Warrior’s burst damage is unmatched, Hundred Blades + Whirlwind Attack are awesome if you like seeing big numbers.
Guardian’s base attacks are stronger by comparison, the Greatsword’s auto-attack hits pretty hard but the skills aren’t as strong as the Warrior’s.
So Guardians have a decent sustained DpS, Warriors have a more bursty DpS. Untraited.
Properly traited, Warriors get a huge boost to overall DpS so even their sustained DpS will be on par with a Guardian’s and they still get to keep more burst.
[Control]
Most Guardian’s control skills come from weapon skills instead of the Warrior’s utility skills (mostly).
This means less damage for the Guardian but more control, and more damage for the Warrior but less control.
[Support]
Both grant decent support, though a Guardian is a more defensive support while the Warrior’s more offensive. Pretty obvious eh.
They can support defensively/offensively as well, but they’re not as good as the other.
[Healing]
Guardians have plenty of small heals that helps keeping you up for a long time, the longer the fight, the better for the Guardian.
Warriors have less of these small heals but they can option to burst heal with traits/utilities. The longer the fight, the worse for the Warrior as he doesn’t have many good options of sustaining his health for too long.
[Ranged]
Guardians sucks. Pretty bad. It is manageable for PvE, but is inviable for PvP (especially WvW). Guardians are heavy melee powerhouses, but anything that goes mid-range makes your power decreases by a tenth.
This means you’re prone to kiting as a Guardian, but this is where player skill comes in.
Warriors doesn’t have it great either, but their ranged options are decent. Weaker than a Ranger/Elementalist or other heavy ranged profession of course, but they do well for what they have.
Heavy single hits or AoEs and pretty long ranges with Rifles/Longbows (traited).
[Utilities]
Guardians have awesome defensive utilities, the best by far being Wall of Reflection. For 10 seconds every 40 seconds, you create a Wall that reflects any projectile that hits it. Traited, it’s 12 seconds every 32 seconds.
This is awesome in WvW and works great when fighting ranged professions as well, as they can’t damage you with anything other than ground-targetted (which you’re supposed to avoid).
I feel kinda bad about this because it feels this skill alone makes up for all the disadvantages the profession has… but it still is awesome.
Guardians can grant Stability consistently for a group, this is a boon that negates stuns and other forms of controls. This is huge in PvP.
Warriors don’t have anything as flashy as these Guardians’ skills, but their attributes/offensive utilities makes up for it in my opinion.
They can also get Stability, but with a longer CD and only for themselves (but longer duration per CD if traited).
They can walk around with nearly permanent Swiftness and Fury, and they’ll always have at least 3 stacks of Might at all times. It’s not uncommon to see a properly traited Warrior stacking up 20~ stacks of Might by himself (dunno if this was fixed).
[Conclusion]
If you want a more defensive profession, with less damage (but still decent damage), heavy melee, weak ranged, low health but with tons of healing, crappy mobility, good group support and Wall of Reflection (this skill is awesome have I told you this?), pick the Guardian. You’re literally a walking mini-fortress.
If you want a more offensive profession with stupidly overpowered burst and attributes, walking around with every offensive boon possible, tons of health but little healing, decent ranged and decent mobility, good to great group support (as it’s all about damage), pick the Warrior. You’re a Juggernaut that can take down someone else (or groups) in seconds if they’re careless.