I cant decide which one(guard or vanguard) is for me
First off, it's important to remember that there are 3 tank capable ACs in the game: Shadows, Guardians, and Vanguards. Ignoring Shadows is going to put a serious damper on your options, especially if you're looking for high performance and interesting.
The primary differences between the tanks can be summed up relatively easily.
Vanguards are the simple tanks. They were explicitly designed as such. They have the fewest CDs, fewest attacks that they have to use, the simplest priority (it's pretty much Stockstrike else HiB else if Ammo 11-12 Ion Pulse else Hammer Shot while using Energy Blast on CD), and negligible active survivability contributions. If you're looking for complex or interesting beyond the basics of tank play, the VG is not for you. I start to yawn when I play my VG as opposed to my other tanks because he's so simple and boring.
Shadows are the fight knowledge and buff watching tanks. They require the most fight knowledge because of how they were designed: lots of self heals and specialized CDs. They require that you watch your buff bar more than any other tank because you have to watch for HS stacks, PA proc activation, and KW charges. They've also got CDs that really should be used and abused as much as possible thanks to their amazingly low CDs. The skill requirement to maintain threat is pretty low, whereas the skill requirement to stay alive is the highest of the tanks, however, they have the highest skill ceiling of all of the tanks, especially when you're using your CDs to optimal effect. A bad Shadow is going to die before any other tank and look bad doing it, but a great Shadow is going to survive situations that no other tank could.
Guardians are the reactive and resource management tanks. Their resource is both the most complex and least forgiving of all of the tanks. Until you learn exactly how, why, and when to play with your resource, you're gonna have a bad time of it. Their survivability is largely passive, though a decent enough portion of it relies in upon using Blade Storm on CD (and Riposte spamming) that they have an appreciable active component. A small reliance on active mitigation means that there is a decent skill ceiling, but not one as high as Shadows get. Unlike a bad Shadow, which is going to get threat and then die, a bad Guardian will simply lose threat all the time.