Putty or Tape

Is it better to use plumber's putty or plumber's tape?

I am putting in new hardware in tub in our bathroom and I in a quandary about what to use, as you can my background in plumbing is limited and I'd like to do it right the first time.

Plumber's putty is used between the drain and the tub. Teflon tape is used for pressure tapered threaded connections. Pipe dope can be used for those threaded connections as well in place of or in addition to the teflon tape.

So, depends on what exactly you are connecting on the tub - the drain to the tub would likely get putty. Most of the other drain connections are likely either glued, or compression fittings and they don't need any additional compounds.

What are you doing?

It is the union joints (?) that connects the water supply lines that I am worried about. I want to make sure they won't leak.

The threads on a union don't make the connection, the face and socket of the union do - the threads compress them together. No tape or pipe dope is required, but some people use pipe dope on the face and socket. Unions work best if the connections are straight and only used once. Sometimes if removed and replaced, there's a ridge on the pieces from deformation upon tightening, and if they don't line up, you either have to tighten it more, or give up and replace. Pipe dope might help in that situation, but I don't know for sure. Don't put anything on the threads, it won't do anything for you. Well, I suppose a drop of oil may make tightening easier, but you don't want it to get into the water, so that's not a great idea.

There is a difference between plumber's putty and pipe dope. The putty is a dough-like material and the pipe dope is a gooey liquid. Teflon tape is used in combination with pipe dope by some plumbers. Personally, I dislike the tape and find, for me, the pipe dope is easier to use albeit messier. As state by other, neither is required for a union although some guys like a small amount of pipe dope on the big nut threads. Of course the pipe ends need sealant. If using tape, if the pipe is for gas, use the heavier weight yellow. Water pipes use the thinner white. Be sure to wrap the tape in the same direction as the threads turn if you use tape.

By asking this question you revealed your need of professional assistance. Please get some help.