Anyone familiar with the difference between UPSes and its VA ratings?

I saw one APC UPS with 400VA and 650VA. The price is not much difference (only 20 bucks) and was wondering if the higher VA will make sense, then I will pay more to get it now than having to buy another in the future. What I'm looking for is to be able to keep power for a wireless AP, a 16 port switch, a cable modem and two computers for about 15 minutes if the power goes down.

Any suggestions?

TIA