I've used both, but am now using VMware. I used Parallels for almost 3 years, it was good. But I did have a few support issues where trying to get help was a pain. Once I had a corruption issue with an expanding disk drive image that could not be recovered. Another time an upgrade of Parallels somehow messed up the default applications under OS X.
The last issue was where right clicking on a file in Windows was causing a delay while my external RAID array had to wake up the hard drives. THis caused a 20 second delay many times a day. This problem only arose after an upgrade. The RAID array had nothing to do with anything in Windows, but it drove me nuts! (Ok, I was already nuts)
The tech support folks were all Russian, there was a language barrier there, they never understood what I was talking about. I was never able to resolve this.
I switched to VMware about a month ago when I got my new Mac Pro.
Parallels gives you more performance.
I thought the Nehalem machines using the latest VMware had changed that? Something about a new addressing table in the hardware?
http://episteme.arstechnica.com/eve/forums/a/tpc/f/8300945231/m/987000447931?r=770002647931#770002647931
I have not done any benchmarks, your mileage may vary!