I disagree with alot of what has been said here.
I looked at my pipe flow tables developed from the university down the road from me.
Coldwater flowtable.
Copper should only be at 5'/second max.
at 1/2 Copper has 3.64 GPM
at 3/4 copper has 7.54 GPM
Pex-al-pex can be used at 8ft/second
at 1/2 Pex-al-Pex has 5.8 GPM
at 3/4 Pex-al-Pex has 12 GPM
The university also has a small piece of pex under pressure @ 500 psi for the last 8 years, it sure has ballooned out and expanded but it has yet to rupture.
Personally all I install is Pex-al-Pex, and am completely confident in the product, I like wirsbo even better and it has better flow rates, but is slower to install and a bit more expensive in my area. About 8 years ago, I did a multi million dollar house all in pex, it was in a ski resort and was in the wintertime. I pressure tested it at 200 psi hydrostatic, and for reasons beyond my control I left the test on for several days. It got really cold and the lines completely froze, when i was able to return, I expected to have to replace the whole system. I brought alot of propane heaters, set them up, plastic'd off the windows and warmed the house. I thawed the whole system and retested it back at 200psi, unbelievably there was no leaks and no damage to any fittings. I kept the house heated and under test for several days at 200psi to make sure, 8 years later and I have never had a callback.
I am completely sold on the product and have used it extensively since, Every other contractor uses it in this area as well. The only place we use copper is on valves, down to the tub spout and up to the showerhead just to make it rigid.
Edit:
Personally I always bring 3/4 pex to shower valves and tub valves, but thats just because I like to be prepared incase the home owner changes their mind about valves and potential setups. I agree with dunbar Plumbing if you dont have the right valve, you wont see an improvement, make sure you get a 3/4 valve for the set up your talking about...
Like Ted said, size your lines accordingly, I only ever run 3/4 to two bathrooms at the max (17 fixture unit is code in my area, 2 standard bathrooms would be 12 FU), then 1" from there, but I have been known to be a bit overkill, and have built a reputation on it.