A real NAS will have far better performance than a USB disk attached to the router. A USB disk on the router will top at around 12-14 MB/s, while a cheap NAS might give you 40-50 MB/s over gigabit. If you intend to store large backups (such as disk images), a USB disk on the router might make those backups be quite slow.
I have limited experiences with those cheaper NAS, as I usually deal with more expensive products from QNAP (which are excellent). You could look at the products from WD - I haven't heard much negative reviews about them so far, so maybe someone who owns one might chip in. The Seagate one (GoFlex Home) however was a headache the one time I configured one for a customer, so I don't recommend them.
I had a few customers with LaCie, and they all died within 2-3 years of light use
Usually the power supply that dies on them.
If for you having the speed limited to 12 MB/s isn't a problem, then a USB disk would be your cheapest solution. For larger amounts of data however, go with a NAS.