Ball valves are much more reliable. Also, if you have the gate valve for the main shutoff, is it against code to put a ball valve right after it, so you have a working shutoff when the gate valve fails?
Gate valves seldom hold 100% with age. Gate valves mechanism is all brass with no washer to deteriorate. I agree modern ball valves are much better for positive shut off of water. It is not against any plumbing code to put a new full port ball valve on your side of the water meter and use it exclusively to shut your main water line off. New ball valve may be made of stainless steel or brass. Carbon steel valves of any kind may not be used on potable water systems anywhere.
I agree with Ed. I have had far more problems with gate valves than ball valves. They can break in either the on or off position which can really be inconvenient. They usually break on me in the off position after performing minor plumbing repairs.I will often use the main shut off at the street if I see an old gate valve. If you are required to use a gate by local codes (I don't know your codes but here in California a ball valve is fine) put the ball valve after it and use it for your shut off.
no as long as the gate valve is there. gate valves are pressure rated and seal better and are made from hardened steel so it will usualy seal when others have a seat [made from brass] that can wear out and leak even ball valves have a ceramic or plastic seal and are made from brass or soft steel
