So far the Versa appears to have good reliability according to survey sites such as TrueDelta. Early build models for the first few months of production (late 2006) had their quirks here and there, but those were ironed out on the assembly line. You certainly couldn't go wrong with the 6-speed for reliability.
As for the SL versus S - I too agree that the SL would be your best bet. In addition to what Billy listed, the SL comes with higher quality upholstery that appears to be much more durable than the S cloth. If you plan on keeping the car forever, this might be one concern. Also, with the S, you get hubcaps, which apparently are not great at staying on the steel wheels. The SL alloys improve the appearance, in my opinion.
Also, the SL has a padded center armrest for the driver/front passenger. Padded armrests on the door panels also come standard on the SL - this is a touch that many more expensive cars lack, it definitely adds to the comfort level.
Like you mentioned, ABS helps the braking performance substantially - not just braking distance, but control during braking.
The standard stereo on the SL has the built-in speed-sensitive volume function, and has a 6-disc in-dash CD changer with MP3 playback. The S has a single CD player that lacks MP3 functionality.
....................
As far as car buying advice, I'll share a few tips. One great website to go to is
http://www.edmunds.com. Here you'll find a new-car appraisal service where you can find out what price most people are buying a particular car for, down to every option you choose. It's called TMV (true market value). The TMV for the Versa isn't especially revealing, however, since it's such a low priced car, there isn't a whole lot of wiggle room on negotiating price.
Nissan currently offers incentives such as $500 cash back for recent college grads (ends 1/2/08), $500 Holiday Bonus Cash (ends 12/3/07). Nissan financing is offering 3.9% APR for 12 to 36 month terms and 4.9% APR for 48 and 60 month terms.
Near the end of the year is usually a recommended car-buying time because many car models get updated around this time of year. So as, say, the 2008 model year arrives on the lots in late 2007, there are many "left-over" new 2007 models that haven't been sold yet. Dealers are usually want to get these off their hands, and you can usually find a better deal on the "left-overs".
As for the Versa, the new model year came out in the summer, so although some 2007s may be out there, it's not quite the same as a model that comes out toward the end of the year. Also, essentially nothing changed from '07 to '08 (except for option package structuring) so it's not like the left-over '07s are outmoded models.
That said, the winter time is also usually a slow time for car sales, especially around the holidays, as you can imagine...so you can usually find dealers are more likely to "deal" with you at this time of year just to get a sale.
Sorry for the novel - hope this helps.