A while ago I was considering putting low-powered 5400rpm drives into a NAS. I was worried about performance, but Tom's Hardware shows us that drive speed isn't the bottleneck, and how slower drives can even beat faster ones.
The main bottleneck in any NAS is the RAID engine. Since many NAS units don't include a dedicated controller, oftentimes the speed of the drive just doesn't matter. If you're using a blazing-fast hardware RAID card in your own custom-built setup, then drive speed might make a difference. But for most consumer units, the controller is the bottleneck.
With that in mind, you can go with slower 5400rpm drives that reduce power consumption, generate less heat, and will likely cost less up front too. Even if you have a dedicated RAID card that could let a 7200rpm drive do its thing at full speed, I'd consider the benefits of low-power drives to outweigh the marginal speed increase you might see.
This chart shows the difference between Samsung 7200pm and 5400rpm drives in various RAID configurations:
Not much, right? So think twice before you drop more than necessary on 7200rpm drives for your backup unit. Check out the link for the full test rundown. [Tom's Hardware]