Me and my friend had this same argument. He said his guardians would wipe my goliaths.
Guardians do 39 damage each hit, and attack slow. They are also EXPENSIVE.
Goliaths cost 125 minerals and like 50 gas, and do 29 damage each missile (this is fully upgraded of course).
Goliaths also build WAYY faster. In the time it would take him to get his guardians i could have a huge army of goliaths. Not to mention that since they both have a good attack range, if you manage to drop a nuke behind the atacking guardians, they will all die without harming your goliaths ^.^
__________________
|