Guardian or Berserker help me decide

If all else fails just flip a coin. Both have their pros and cons and you can't go wrong with either.

or you play both class go with your 1st mine in pick your class max it out then make other one of your 2nd class in then you will which class

best for you.

I'm not sure picking a character based on how fast you get to cap is the best idea, more often than not you'll wish you picked the class that seemed the most interesting to you. Go with what you like, and the rest just falls into place.