Some would agree with the Dealer, only because fixing the one head could unbalance the engine and cause the other side to fail as well. Besides, the dealer would be out several thousand dollars for the new engine.
Since you don't know exactly what is bad, you might as well take the head off and see what's up and repair it. It could be a burnt/bent valve, pushrod, sticking lifter, or head gasket. The key is to remove the other head as well, even if all you end up doing is replacing the head gasket. More than likely you're going to send the heads to be resurfaced and cleaned, etc. Again, do them both, not just the one that's bad. This should keep them balanced and reduce the probability that the increased compression will cause problems with the lower engine, due to wear.
__________________
Good Luck...
mj
'01 Ford Ranger 3.0 Super Cab, K&N Air, MaxFlow Exhaust, Oversize Tires 
'93 Ford Taurus 3.0 (Ugh! What a MONEY PIT! Traded it in for the '01 Ranger) 
'87 Ford Ranger 2.3 Super Cab (Owned it for 16 years until some clown t-boned me in it!) 
'92 Ford Aerostar 3.0 (Gave it to Ex in Divorce Settlement) Hehe...
'74 Ford Maverick 250 ci (Great car! 110 mph on Autoban! Traded it in for the '87 Ranger)
'66 Ford Mustang 200 ci (Screwed up and SOLD IT!!)  | 
