Build quality, longevity, etc. If you just need something for consistent basic performance, it makes more sense to get something that isn't shit and won't fall apart in six months, since any modern laptop is overkill for basic performance for at least the next 3 years, if not much more. Normally, I'd be arguing against purchasing a Mac since they carry a huge markup, but if you insist on the ultrabook form factor, there's no way around the fact that Apple's competitors are currently producing absolute garbage. And the new MBA is going to have that game-changing display. Anyway, I'm just trying to be helpful. It seems like most people are doomed to retroactively justify whatever purchasing decision they've already internalized, which is definitely the wrong way to go if you want to buy a good laptop (or a good anything for that matter).
I personally use an old ThinkPad with a bunch of spare batteries, but I've been considering the T520 as an upgrade because of dat 18-hour battery life and relatively great build quality. I'm waiting on Ivy Bridge to see what kind of battery life improvements it can offer before I pull the trigger.