Dell battery's come with a 1 year warranty. Nearly all dells get 3+ on their batteries, unless you have the incorrect power management configured.
What dell computer you looking at?
The Vostro is a tank, large frame, and inexpensive (relatively speaking)
The 13 and 14 inch inspirons are a decent machine, with a mediocre 128M video card.
I have the XPS 1530 which is very nice. Little more spendy the the inspiron.
The mac air is nice and portable as is the 13 inch inspiron for roughly 40% of the air.
The XPS is built better than the inspirons as they should be for the extra one pays. Latitudes tend to be built to support business customers, cost a little ore but to me worth it.
Hardware wise as you move up and through the dell models the hardware does improve.
Vista, well I feel its a HOG in every aspect. Recently loaded Windows 7 and I like it far more than vista.
Its Dell's seasonal time where they start announcing/introducing their new lines for the August/September time frames.
Every dell laptop I have purchased I buy with complete care. Everything but the battery is covered. And I have used it on countless occassions to tomy negligence or clumsiness. I am rough on laptops so its well worth the cost and in home service. Dell's support has greatly improved as well.
Cost wise the mac will most likely be more. Aesthetically, the new models are starting to be show on the dell site, CNET and so on. Dell, sony and HP will be focusing on looks as laptops are as fashionable as a new pair of sneakers/shoes. They even make that comment in 2 of the PC commercials how much nicer Macs look
Dell's new latitude mini is a sneak peak of whats to come.
If you do decide to go with dell take a look at
www.gotapex.com , they list Dell Deals nearly every day. I have no affliation with gotapex other than I have used their site to learn about the deals.
Gie both mac and dells a good look over. I know many hear will tell you how crappy dells are. Having used most of Dells laptop lines ove the past year, I am not going to kid you saying the $400 dell is built as well as the $2000 dell or the mac book pro.
Good luck