Where are you going that the tool costs $300? Second question, which pex are you talking about? Uponor/Wirsbo PEX with the expanding tool or PEX with crimp rings? The "Quick and Easy" tool for Uponor is $300+ (gotta love propriatary equipment) but the tool for the crimp style (like PolyB) is only $100-150. Plus you can always rent them from most plumbing wholesalers and some box stores like Home Depot. As for the Pex vs Copper argument... unless you are skilled at soldering and can guaranty that you won't burn your house down, Pex is faster, easier and getting to be cheaper to install.
I cannot say which PEX I would use. I will have to do some more research on it.
As for copper, I have done quite a bit of copper sweat. I used a fire pad that I bought at HD. It worked very well. I am also very careful to plan where my solder joints will be. I won't put them right on joists/studs, or in other places where it would be difficult to control the torch.
I do as much work on the bench as possible.
Like I said; I'm going to have to do more research on PEX before I get into using it for anything major.