As far as pure tank, Guardians are technically superior due to better armor and higher HP. Zerkers and other tank classes are excellent tanks as well. Priests throughout Norrath will all tell you that guardians require less healing, but ultimately, any tank class can effectively tank 98% of the day-to-day creatures in the game. Only when you start raiding (and reach end-game) will this become an issue. For your daily grind or dungeon crawl, any tank will do and it comes down to gear/level when deciding on who will be MT.
I chose guardian only because our guild needed an MT (main tank for raids). For soloing and duoing, zerkers/pallies are more fun and just as effective. Only when that big bad dragon needs to be tanked will the clerics start calling for the guardians. For everything else, the other 98% of the game, any tank will do. Pick the one that sounds more fun to you.
Only if you are intent on being the MT for a guild (and soak up first dibs on all raid tank gear), should you play a guardian. Only if you are a hardcore, no-life, must be uber player will you be required to pick a pure class like a guardian. Chances are that you will be in guild that already has an uber tank or two, so evaluate your ability to play the game for extended hours and decide if you are willing and able to dedicate yourself to the grind required to be end-game. Otherwise, pick a fun class and enjoy yourself.
____________________________
Duke Ardnahoy
Noble Lord Protector of the 70th Crusade
Fennin Ro - EQ I
Ardnahoy II
Fearless Guardian of the 40th Tour
Mistmore - EQ II