Optical heart rate sensor vs bioimpedance sensor?
The optical sensors (photoplethysmography) rely on the change of volume of blood in a digit or earlobe (based upon the pulse) changing the light absorption characteristics as detected by an LED/photodiode pair.  They're pretty easy to use, and hard to get wrong.

Bioimpedance plethysmography is much the same, but relies on a very small electrical signal provided at one point on the body and received at another.  

It's pretty well described in 
A Bio-Impedance Measurement System for Portable Monitoring of Heart Rate and Pulse Wave Velocity Using Small Body Area Min-Chang Cho, Jee-Yeon Kim, and SeongHwan Cho, in Proc. IEEE EMBC, 1997, pp. 2072–2073. (though this will be hard to get outside of a university)

My understanding is that the bioimpedance measurement is trickier, as electrode placement becomes pretty important.