The best advice I was ever given was figure out what software you want, then choose the machine.

I like mac's but, unless this is going to be a desktop replacement, the most important spec is heat generated. Especially in the class you are looking at.

They never spec that so the only way I've found is to look at the machines label and see how many watt's it consumes.

The added advantage of a low wattage machine is that it will run longer on a given battery.

A 1.6GHz machine is fine for anything that isn't number crunch intensive.