Each computer gets to keep its own monitor, which gives you more visual real estate while slashing hardware clutter. Here's another bonus: you don't have to physically switch between systems anymore, you can can stay put and let your mouse do the walking.
If desk space is no issue, Synergy is going to be a much more freeing solution than a KVM switch, which, in an important reversal, uses only one monitor to control multiple computers, won't let you see data side-by-side, and requires extra hardware. As another perk, Synergy merges your clipboards into one so you can copy and paste among platforms.
Synergy isn't limited to Macs and PCs; it can also work with two or more computers on the same platform, and there's a Linux build, too. Synergy involves a little more setup than your average application, but the results can be well worth it. CNET Editor Jeff Bakalar's video (above) has plenty of useful tips to get you started.