Since you're going to have to support these machines and you've already figured out that their speeds are adequate, you should probably buy whatever you feel will be the easiest to deal with from a maintenance and warranty perspective. To me, that points to Dell (in a landslide) rather than ASRock, if your choice is strictly limited between those two.

BTW, the only reason that newer versions of operating systems and applications feel like they're maintaining speed is because hardware is getting faster. If the applications were truly maintaining their performance and usage requirements were truly as limited as they used to be, then applications like Word would be lightning fast on current hardware. Photoshop feels just as slow today as it did 10 years ago.