Pex is the best. Easy to flex, buryable, freeze proof, can run hot or cold water. The only real downside is the tool it takes to properly use fittings costs about a grand. There are other "homeowner" type fittings that are acceptable and that tool only costs a hundred or two.
Most people use PVC, as its the cheapest, real easy to work with, easy to modify/add fittings, etc, lots of parts available, did i mention cheap? You do have to bury it and/or insulate it good because it will crack if frozen with water in it, but its slightly more forgiving then copper, plus you can get it in more sizes.
Copper is probably the worst. Hard to work with, fragile to punctures, suseptible to freezing and bursting, doesnt bend.
Not ideal to share a tranch, code usually says they can share a trench if they are 12" apart. I'd put the water down lower, and one good idea with any underground tubing, especially water supply or waste is wrap its length with a wire that sticks up at the beginning and end of the run, so its easy to locate later if you have to. Or if you want to go super budget, another neat trick is after you shade the pipe (thats where you bury the pipe you just ran in sand or dirt for a few inches so that once you finish burying the trench the bigger rocks dont puncture the pipe or create voids in the soil) run some one inch neon "tape", kind of like caution tape, on top of the shaded pipe just below the surface. That way if you or someone else is digging they will hit the tape first and not the pipe/wire.
The reply above is misinformative, be wary. I'm a licenced plumber, gasfitter and drainlayer with more than 30 years experience in the trades.
Copper with the joints silver brazed is best hands down, it's forever, it's malleable, strong and will never rust, warp, split, rot, or freeze if you bury it below your local frost depth and put fine fill in direct contact with the line, avoid jagged rocks.
Correctly pressure rated alcathene, poly, pex and as a last option PVC also have their uses. Try for continuous runs without joints if using one of these mediums. Again backfill the line with fines first. Sand is often the best choice.
Trench sharing for services like water and power makes sense. The Australian Standard requires a separation of 200mm in most circumstances. Power is usually the deeper service as the standard requires greater cover for buried electrical supply. Check your local standards to avoid compliance issues. Use a licenced tradesman if that is a local requirement. Save tonnes of money by doing all the digging and backfilling yourself.
Use approved marker tape for each service as described above, that part was not misinformed.
Yes,I totally agree with rhymeswithorange. I have experienced with Pex. It works really good. It is the best. It is easy to bend, buryable, freeze-proof, water can get hot or cold. The only real drawback is the tool you need to properly use a large soft cost.
I use Pex and copper and find they both have their applications. I own my own Pex crimpers and they were not even close to a grand. The 1/2" crimper was $120 and the 1/2, 3/4" combo that uses a pair of vice grips was $60 @ HD. When I had all my trenches dug, I put my poly pipe water lines on one side, a separate poly pipe on the other side (3 feet apart) for future wire runs of UF cable. I put a few inches of fines over the pipes then ran 4 pair direct bury phone line down the middle to help locate later if needed. My advice to anyone is to throw as much pipe and cable as you possibly can into the trench before backfilling. Saves a lot of headache later.
There are different kinds of PEX connections, just as there are different proprietary methods to making PEX. All manufacturers have a "crimp" or "cinch" style connection which work, but for me I prefer the best that is available. Wirsbo/Uponor Makes what is known as PEX-a. They have a fitting method knows as "cold expansion" in which a thickened band of the same material is slid over the cut end of the PEX tube in preparation of sliding in a fitting (tee, reducer,bend, etc). The tool expands both the tube and the band, then you slide the fitting in and they immediately recompress back to the original memory of the tube. What I like about this style of fittings is A) the band and the tube are made of the exact same material, so they expand and contract at the exact same rate; and B) by expanding the tube around the fitting, the inside diameter remains the same, so you get no loss of pressure/volume no matter the amount of fittings. This battery powered expansion tool costs about $500. They are rare to find used, which means if you buy one to use for your house, you could turn around and sell it at the end of your job for minimal loss. I think PEX is great.
I also second the advice of adding extra pipes and chases into existing trenches. Also, after you shade the pipes with sand or fines, I like to run some caution tape or high visibility ribbon on top of the sand, over the pipe, the entire length of the trench. That way in the future if you or someone else is digging around, it can save a huge amount of headache/work/cost from not severing a line!
I like metal. I especially like copper. Copper is naturally antibacterial. It will out live you. Copper and concrete do not mix however. Concrete will chemically eat copper. Pex tubing is my #2 plumbing choice. I know plastic has some longevity issues that we are only just discovering, so I have to wonder about the long term survival of the otherwise awesome Pex tubing.
I am an experienced water and gas plumber, and I would suggest that you use copper for water pipe. It is still the best one to use. I would not suggest using Pex, PVC or poly for you since you might not be able to handle these properly. Either ways, you just need to get an experienced water plumber to do this task for you.