I personally bought VMWare Fusion and haven't had any issues. You're going to get parties for both sides saying why theirs is better than the other. My recommendation, use the trials and find out which one you like better, or which one suits your needs the best. I put the links below for you. A third option is the free option and use Sun's VirtualBox.
Ive been using Parallells over the last week in SL. Excellent integration, works really nicely.. Im using it with Windows XP, which I really need to just run a legacy financy package thats not supported on OSX.
I prefer parallels with my SL macbook pro. I installed both XP SP2 and Windows 7. Try downloading the new parallels 5 beta, you will surprised. Its speedier, handles memory better and I havent noticed a single spinball when using it.... and it also supports the aero interface of Win7. Its very cool.
They are practically the same for your needs. It's only when you need more than 4GB of RAM in your VMs that matter. And for that Parallels is slightly better.