Well, this is easy. If you have v5, I can't really recommend buying v6. I have found no appreciable useful difference between v5 and v6. Perhaps it is better in expanding disk performance, which I don't use. Actually stopped using expanding disk because fixed size was way faster. Then I moved to virtual on bootcamp.

Don't get me wrong. Parallels is a great product. If you are buying it for the first time, there's no reason not to get v6, but I'm not sure I'd pay more than $10 for an upgrade and that just to get to ride the upgrade/patch train.

The Parallels Experience

A Freeway of Delight

• Wow. Taskbar and the Dock are one. The start menu is right there! And I can Cmd+Tab across Mac and Windows apps! Double-click an Excel document from Finder and Excel launches! Launch Mac programs from Windows.
  
• Ooooh, coherence mode. My Windows apps are running in, uh, a window!
• Brilliant! I can always use Cmd+C and Cmd+V, even in Windows! It's so clumsy having to remember Cmd+C vs. Ctrl+C.
  
• Awesome. My %USER_PROFILE% folder points out to the Mac disk!
• Cruising through the configurations, man, this product is built by passionate+crazy smart people. Features like disk resizing with guest resizing... nice. The incantations and shenanigans you have to go through to do this on a "mature" product like VMware made me recommend a while ago to just make vdisks crazy big, always.... bigger than the host disk even. Now, I don't have to. Turns out, these are the crazy smart people behind Virtuozzo, the OS-layer virtualization system used by a lot of hosting providers.
  
• Heh, heh, heh. The features of Aero that I hate don't seem to work, but the ones I like do.
  

The Alley of Sorrow

• Oh, my dream of keeping all my projects on the Mac side are dashed. Even after adding \\.psf\Home and the network drive mapped to it (Z:) to my Trusted Sites list and using caspol to make .NET trust them, I still can't really work with Visual Studio projects off the mapped disk or host sites using IIS from there -- I think this is due primarily do lack of ChangeNotification support. ASP.NET really wants to know when a dll or web.config changes. Visual C++ totally refused to build even a console project there. If I had one wish for a future feature, making this work would be it.
  
• Alas, I had to kill the beauty of Cmd+Tab across Mac+Windows because I needed to be able to use the function keys in Visual Studio. Boo.
  
• Minor hiccup: I set up another virtual machine to run Windows XP and IE6, because it is the most awesome browser! Anyway, running a second VM made things a little weird. I was expecting a second start button on the Dock, but instead I had to switch between them. Not a huge deal, given I was just going to run IE6 on the other VM, but the main thing was the networking totally didn't work when the VM came up the first time. Not sure why, but I could ping the gateway, but DNS didn't seem to work.
• I like the fact that I can access the Windows disks from Finder, but, it looks like that only works when the VM is running and didn't seem to work reliably. How sweet would it be if I could work with Windows disks anytime, but that, of course, would require Parallels include a full NTFS driver.
• The audio and video playback doesn't work quite perfectly, but given I run iTunes and surf mostly on the Mac side, this doesn't present a big problem for me.
• Another minor feature request: It would be nice if it could virtualize the iEye camera a little better. Allow all VMs to see it as hardware and the first to turn it on gets exclusive use of it until they turn it off or the VM is killed.
• I had this problem on bare metal too, but Aero seems to switch on and off. Sometimes, I'll have translucent window frames, other times opaque.
• I expect that when I shift+click a Dock icon for a Windows app, that it should launch a second instance. Heck, Parallels should add that for Mac apps. Ever try to run two instance of Calculator?
  

Conclusion

Caveat

VMware workstation allows a maximum of 950GB. That's quite a bit more than VirtualPC 2007 allows. So what's my advise on sizing? As before, do the max.

Windows Server 2003R2 install makes 1,637,548,032 byte .vmdk. Still under 8% difference.

"But I don't want to pay the extra 117MB." Really? That's pretty cheap compared to what the space required to install a partition resizer later along with your time.

"But I'm only going to use this virt for a couple of things." Great. Then when you are done and delete it, you'll get a full refund of your extra space.

"I don't want my virt blowing up my main machine by causing it to run out of disk space." You could enable disk quotas.

Don't be a cheapskate when VirtualPC 2007 asks you how much disk space to allocate for your machine. Do the max! I just did a comparison of three identical installs of Windows Server 2003 on a 8GB, 64GB and a 132GB (which is the max) disks. The difference? Less than 2.5%. Here's the numbers:

8GB - 1,514,532,352 bytes
64GB - 1,537,721,344 bytes (22MB more)
132GB - 1,552,535,040 bytes (36MB more)

But why pay that extra space? If you ever had to grow one, you'd know why.

But why 132GB? I don't even had that much space on my drive? Sure, but the beauty of a virtual disk is that you can copy it off to another machine.

That's a key point: Your virtual machine may outlive your real hardware.

I'm gonna guess the difference on VMware is similiar. [Edit: Yep - link]