Buy the 50" 720p. You won't even notice the extra pixels on the 42" set. And actually, it will make your standard def viewing that much softer on the 42".
42" from 10 feet is a little on the small side, too.
Lastly, I don't know if you're factoring this into your decision to buy, but I have found that the difference between Blu Ray and DVD on a 50" plasma from 10 feet is pretty marginal. On the audio side of things, Blu Ray rocks my world. But the screen's got to get up in the 60 inch territory before today's plasmas start making DVD look soft enough to justify the upgrade to Blu Ray on the video side.
Go BIG and don't look back. You'll regret it otherwise.
