GPS speedo

These two seem popular, but I have a hard time dropping even only $65 (much less $150) on a device just to measure speed... thus far I've used an old iPhone LOL

SkyRC GPS $65 -

Garmin Foretrex 401 $150 -

I have an old Garmin eTrex Legend I just dug out, but I don't know how quickly/well it can measure changes in speed, I may give it a few runs just to compare and see. I'd rather have that at risk then even my old iPhone.