Nope, no difference. HDMI is more or less "built" on the DVI video standard. Meaning video through HDMI is the same as video through DVI. You can even buy HDMI to DVI cables that require no modification to the video stream.
You might have an easier time buying long, cheap HDMI cables than DVI cables, and, as you mentioned, HDMI includes sound. HDMI is also a bit easier to manage than DVI. On the other hand, I'm not sure if HDMI will work with resolutions over 1920x1200. While a dual-link DVI connection can work with resolutions up to (at least) 2560x1600.
But, a 42" plasma TV if it supports 1080p, probably can't handle anything over 1920x1080 (or possibly 1920x1200). If it's 720p (this includes "1080l" -- if there's no 1080p support), then it probably won't handle anything over 1280x720, or possibly 1344x768.
All of those are well within the capabilities of HDMI. So you're fine with either cable.