I personally find VMware Workstation seems a lot more professional.
Virtual pc works, its free, its not a bad product, but if you have the money to spare, I think Workstation just has a few killer features that are much better.
Virtual pc, it seems good for just running one or two virtual pcs, but if you want to do something advanced the UI becomes a mess. VMware has some nice features for multiple virtual computers, I have no less than 40 on my laptop (using external hdd) and I usually use around 7 or 8 during some technical demos, the interface stays really nice and it has some great presentation features to switch between the machines..
I do like Virtual pc and have it installed it for a few people who wanted to run a old game or application that was under win 95/98, it works fine and good... but Workstation just feels more of a business class product.
The only annoying features with Workstation are Blue screen frequently (but avoidable if you know where they are) and despite me bringing it up time and time again, they blame my configuation (nothing out of the ordinary and I have had it on multiple computers), and it creates a load of NIC objects. Apart from that, I love it.