Of the 3 pieces you are all talking about, here is my order of which leak the most often on main lines or otherwise 1. Compression repair fittings 2. Unions 3. Telescoping slip fixes I personally use the slip fixes, and on mainline I get it extended out as far as possible. Now the thing is, any of them will do the job properly and probably not leak. But, they all have to be installed PROPERLY or they will leak. I don't like having any extra thread joints in a mainline if possible which is why I use the slip fixes. But I don't see anything over 1.5" pipe.