Pex Water Piping
I redid my house with Wirsbo PEX some 6 years ago. About a year after my installation, I noticed that Wirsbo had come out with a line of plastic (polysulfone) fittings in addition to the brass fittings. So I asked my plumbing supply house, where I get my Wirsbo tubing and fittings, what their opinion of the plastic was.
They told me that the plastic was a better choice than brass in acidic water conditions (I have relatively acidic well water) because the brass corrodes over time in acidic conditions. It may also corrode under alkaline conditions, I don't know, but I have noticed that a couple of brass fittings I have removed in the last six years have shown signs of internal corrosion. None of the plastic fittings has had any problem.
A couple of words of advice, based on hard experience. Sooner or later you are going to have to either add on, replace, or modify your plumbing. Leave enough straight pipe available between fitting so you can easily add a T if needed. I have one section of pipe where I did not do this, and it is a royal PITA to get in and modify the line.
Also, if you ever remove a fitting, do not plan to reuse it, sooner or later it is going to leak. Best to plan on replacing a full section of pipe, less headache in the long run.