Should plastic or copper pipe be used when renovating a bathroom? There are advantages and disadvantages to both.
First Copper - it's been in use for plumbing for well over 100 years and is time proven to be reliable when correctly fitted. If it has to be exposed in places it looks good with or without chrome plating compared to plastic. Compared to plastic copper pipe in early 2019 is a similar price per meter however this is constantly changing with the price of raw materials. Copper installations do take longer simply because of the time it takes to cut and bend the pipe and solder the fittings as required for a particular job. Being less flexible than plastic, copper has a lower capacity for dealing with expansion in freezing conditions. The big material price difference between plastic and copper is the fittings. Where as a copper solder fit can be as cheap as 10p a plastic push fit will be upwards of £1.4 even when bulk bought. Have a more complicated plumbing requirement and using dozens of fittings this adds up.
Copper plumbing has one long term disadvantage compared to plastic though that plastic pipe does not suffer from at all and that is what is referred to as both Galvanic corrosion and de-zincification. This is only a problem if your property is in a very soft water area. Most people are aware of the problems with hard water and limescale but soft water can be just as bad. Soft water acts as an electrolyte. Since brass is an alloy of copper and zinc, what literally happens is the soft water strips the zinc from the brass leaving a fittings covered in what looks like salt. You can buy de-zincified (DZR) fittings but these still contain 15% zinc and will in the long term be damaged. I have been into properties where the installer has not known this and used standard fittings and found every brass fitting effected and needing replaced. The other effect that happens is galvanic corrosion. Though compared to other different metals that experience galvanic corrosion because brass contains mainly copper and the pipe is copper the effect is not as notable as DZR but does still occur over time where one wrecked brass fitting is replaced with other. Eventually and I do mean over timescales of 20+ years the copper gets thinner and thinner until it fails. With this in mind in areas of very soft water the use of brass fittings should be kept to minimum and DZR fittings used when absolutely necessary, such as isolation valves and stop taps. You should be aware that virtually no shower valve or tap manufacturer provide DZR fittings so whilst the the rest of the installation can be done correctly any brass tap etc will eventually be effected.
One solution to soft water problems for copper and brass is to use plastic pipe instead.
Plastic pipe is much quicker to the point where in most bathrooms the savings due to the less time taken out weigh the extra material costs. You may have heard of horror stories of some people having horrendous floods with plastic pipes. Fitted correctly and pressure tested before being put into service plastic pipe is reliable. In fact compared to copper it has a better capacity to deal with increased pressure before bursting, and also from freezing. There has been discussion about the long term durability of the plastic push fit fittings which has rubber o rings and seals inside them and their longevity with unverified claims they only last 25 years before failing. If this is true then every house with plastic pipe plumbing will need to be re piped eventually costing the home owner a small fortune pulling up floors and taking down walls to access all the fittings. From an installation perspective there are two disadvantages to plastic one minor and can be overcome the other not so much.
The minor disadvantage to plastic pipe is the installation of inserts into the pipe before installing the pipe into a fitting. This effectively reduces the internal diameter from 15mm to 10mm thus reducing the flow rates. It is more of a problem in a traditional installation with gravity fed supplies from hot and cold tanks. With the already low pressure and now the flow rate being reduced the flow rates from showers and taps can be very poor. This is very true when the taps are plumbed in using flexis. This is also a problem for low pressure mains pressure areas. Copper does not suffer this problem. There is a way to get around this and that is to run 22mm pipe from the supply as far as possible to the shower etc and then as close as possible reduce it to a 15mm but really this is not the most practical thing to do.
The major disadvantage to plastic compared to copper is vermin. Plastic pipe is barrier pipe i.e twin wall but it's not enough, vermin love to eat through it resulting in potentially enormous damage to a property. If you can guarantee that vermin are not a problem for you then this won't be a problem. For this very reason alone I have no plastic pipe plumbing in my property, but we also don't live in a very soft water area. You should also check with your home insurance provider in the small print that your insurance is still valid in the event of water damage due to plastic pipe failure. The insurance companies have had so many claims as a result of improper installations or vermin damage that they may not pay out on a policy if there is damage especially rot damage as a result of improper plastic pipe plumbing installation.
One final note to make, plastic pipe from the two UK's biggest plastic pipe manufacturers is constructed from polybutylene which is banned in the USA for plumbing issues following numerous lawsuits relating to failed pipe causing water damage to property. Do a search for this issue there are plenty of websites with further details. Needless to say the conclusion seems to be that due to the higher chlorine content of drinking water supplies in the USA, the chlorine attached the polybutylene eventually causing it to fail. Here in the UK our concentrations of chlorine seem to be much lower, how much lower though isn't obvious since not all water authorities both here and in the USA have their dosing concentrations for comparison. As far as I know there has not been one issue with this so far causing plastic pipe failure.