I used to use copper for supply lines.
I've also seen iron pipe, CPVC, rubber hoses and PVC used.
I do a lot of remodeling.
I hate leaks. I've developed a solution. It also requires less fitting as the tubing is fairly flexible. It also makes it easy to identify a hot or cold line by the color.
Use pex lines, brass and copper pex fittings, and valves. Use the wide copper crimp rings. Fit the pieces and crimp together as you go.
Making leakproof connections with PEX is easy.
1) Cut the tubing with a PEX cutter so the ends are square and without burrs.
2) Place a collar over the end of the tubing where it gets crimped.
3) Insert PEX end into tubing as far as it will go.
4) Slide the copper ring about 3/16s of an inch and squeeze slightly with a pliers to hold it in place.
5) Crimp the collar.
You can use the gauge that comes with the crimping tool to check the connection.
I have yet to see a PEX connection fail. Ever.
Use 3/4 inch line (and fittings) and you won't have to worry about figuring out what size you "should" be using. The few bucks you save by using both 1/2 and 3/4 is offset by the hassle of doubling the amount of fitting inventory you will need.