Western Digital 500GB My Passport™ Essential™ Portable Hard Drive $102 After Coupon Slickdeals.net is your friend...
With your budget, you could get two of these bad boys!
I think the biggest difference between some external hard drives is the 3.5" HD vs the 2.5" HD.
The 3.5" is a regular hard drive that's usually put into a desktop computer and the 2.5" HD are for laptops.
You would want to get an external that's a 2.5" because those are smaller and are supposed to hold up better for portability. 3.5" HDs were not originally made to be 'portable' because most people don't lug their desktop computers like they do with laptops.
Like everyone has said, it's not that the externals die more quickly, it's because they are prone to more bumps and drops that it causes them to malfunction. Essentially the 2.5" HD in your laptop is no more studier than a 2.5" external.
Also, the lifespan is NOT affected by the amount of storage. The only thing is, if you got the 1TB instead of two 500GB externals, and it crapped out, then you'd potentially be losing 1TB worth of data. With two 500GBs, if one were to fail, you'd still have the other HD with data.
Sounds like the way you'll be using your external is the way I use mine right now. I would suggest the external I gave a link to. It's small, quiet, plug and play, only requires USB cable to transfer and power, and relatively affordable.
If you really want to know if WD or Seagate is better, there's a small discussion in the link where people discuss.