So may I rely on taking noticeably less dmg and give some advantage to my raid if i go Guardian instead of Vanguard?
Assuming you're playing your Guardian properly, you'll take more damage on an equivalently geared VG compared to a Guardian. VGs also have fewer CDs and functionally weaker CDs than either of the other 2 tanks. The only advantages that VGs have over the other tanks is that they are *ludicrously* simple to play and that they have the most level incoming damage (because of their reliance upon massive amounts of shield/absorb and armor as opposed to defense).
24k is more than enough hp. Assuming you're not wasting itemization on accuracy and we're talking about equivalently geared characters, you're going to have more mitigation thanks to that lower hp: you'll take less damage from mitigatable sources and have more than enough hp to survive those attacks that you can't mitigate.
and the the dmg they deal is elemental)
VGs are actually the worst tanks for elemental damage: they have the lowest I/E DR (19% compared to the 20% amd 21% of Guardians and Shadows respectively) and Shadows and Guardians both have direct damage counteraction mechanisms (Guardian barrier and Shadow self heals) that allow them to mitigate all types of damage.
and it's a pain for healers, and having more "save buttons" and more defence doesn't help at all.
The only really useful CD that VGs get is Reactive Shield (Smoke Grenade should be used on CD and Adrenaline Rush, like all of the self heal CDs, is of marginal use as a CD). Guardians get Warding Call (which is *better* than Reactive Shield) and Saber Ward (also better than Reactive Shield). Anyone that thinks that you can't get more out of those 2 CDs than a VG gets out of Reactive Shield simply doesn't know how to use their CDs.
I'm gonna bet that your guild is relying upon their experiences with random pugs as opposed to real analysis of the classes. A lot of poorly informed guilds see VGs as the only effective tanks because it's really hard to screw up playing one so, in randoms, VGs *seem* like they're better than the other tanks. In the hands of a good player, all of the tanks are going to be equally good. In the hands of a *great* player, VGs are going to be the worst (for much the same reason that they're hard to screw up: they don't rely on player skill).