Hello vlk!

I see PROs and CONs about the two solutions.

Calculated Sun Ephemeris. 

PROs: 

more reliable (the sun will be exactly where Sun Ephemeris say)

CONs: 

you need to know the right position of your panels (by servo motors, runend switches or similar). 

you need Internet (or some kind of clock) to know what time it is.

Clouds. If there's a cloud in front of the sun, the max light can come from the blue sky and not from the cloud in front of the sun 

Photodetectors  

PROs:

always orientate to the position in the sky where the maximum light come from

don't need any reaction from the position of the panels

no need for Internet or other Real Time Clock

CONs:

Can be betrayed by some different light sources (mirror reflexes, strong lights, ...)

Can become unstable if the sun is cloudy or early in the morning or in the late afternoon (a few counter-measures exist)

Let me know if you agree.

Bye,

Guido

why not to use calculation from location and time?