At the same price I'd take VX. The transit into Manhattan (mentioned above) is a wash on mass transit and $20 different in a private car. I like the seat better on VX (duh!) and drink equally well on both. The food service on VX is neat but I think the food is actually generally better on CO. Still, at the same price that seat would win every time for me. I'm guessing that you'd access the PC in LAS with an AmEx PLT or *G card so you can't use it for the VX flight, right?

But for $350 I'd take CO every time. The difference on the 4 hour flight just isn't worth that much.

Regarding lounge access in IAD, you can always pay for access to the UA RCCs there, at which point you'd be welcome to pay for drinks on top of the $50/person to get in the door. Not such a good idea, IMO.