If you do a bunch of burning, it's sometimes good to pick up a spare drive on sale, just in case.
The 7S offers better speed on a wider range of media, but slightly higher error rates. (especially on RW media). At 52x, the error rates are lower than the 6S. The error rates on CDR are not significantly higher. I use my 6S for RW and 7S for CDR.
The 7S also offers the shorter case, which improves air flow in the box. Having 2 burners is always more fun, if you put them on separate channels you can burn to each one independantly and crank out twice as many discs in the same time.