I only have the experience with the Wahoo, both the S+C sensor and cadence only RPM. Both are great and have been very reliable with my iPhone.

Looking at the topeak one, it seems to lack the "arm" for the speed sensor - this might make it hard to line the main sensor up with the pedal and wheel magnet and get it close enough to register consistently for both for some bikes. I know the bike I put it on wouldn't have worked with out this.

One additional bonus with the Wahoo - the option of rubber band attachment vs zip ties is nice if you want to move it between bikes.