My feeling is that saving a few bucks to get a cheap jack isn't going to add up to much when it fails and you're injured or worse. Don't get a cheapo jack. Think about how long you'll be using it. This is an investment, not a dinner out. Cheap out on the wine, don't cheap out on the jack.
Here's what I got. High quality, heavy duty but light enough that I can lift it, slow profile good enough for all but most slammed cars: OTC 1532 2-Ton Capacity Aluminum Racing Jack http://www.amazon.com/OTC-1532-Capac.../dp/B000XSCEBM
And I agree with the above, this does not preclude sensible use of jack stands. I wouldn't stick my head under the car without it being on stands.
Edited to add:
Also consider where you'll be using the jack. If it's solely for garage use, go ahead with the iron/steel jacks. But if you're planning on throwing it into the trunk or trailer and taking it to the track, get a lightweight alloy, unless you're Dog.