I wouldn't say there is a 'best' configuration. All depends.. available funds, goals of the project, etc. Some more ramblings from me..
- The higher you go in system voltage, the smaller wires/less voltage drop you will get. 12v and 24v systems are pretty common, higher voltage not so much so. Things like charge controllers start getting expensive when they are not so common. I would stick to 12v or 24v.
- Parallel batteries should generally be of the same type and age, otherwise one may start overcharging. Lots of info about this on the web.
- Two series 12v 17ah batteries is 24v @ 17ah or about 408 watt hours, that is a lot of power for a pi! If your Pi setup takes 3 watts, that is about 5 1/2 days of runtime. You can even just use one 12v battery. I say use one or two and use the other batteries for something else.
- A 13w solar panel will likely not keep a normal Pi running all the time. I had a setup with a 20w panel and it just barely kept up a Pi 2B, camera and wifi setup (about 2.9w) in 24 hour operation. I had less of a reserve using 7ah battery.
- Make sure you use a charge controller. 13w is enough to cook your 17ah batteries without one.
I have a small writeup about my setup, might help you.
http://www.allspectrum.com/mopower/sola ... solar.html