Whilst you can just reach them with a 35ft pole on modern two-storey houses (I do occasionally as well with my 35ft) there will be many occasions when this would not be enough.
If I was targeting Solar panel work I would aim for a 42-45ft pole as this would give you the comfortable reach needed.
next question is ... supermax or sl extreme 
Might come down to price - How many are you doing each week? Could you justify the expense of the Xtreme?
As I see it from our range you have three options:
1. Xtreme47
2. Smax45
3. 8x Super-Lite Modular extensions with top gooseneck adapter
Of these 3 options the Xtreme would be the most user friendly and useable on most other work. The SMAX45 would be the most flexible, but this could be improved by not fully opening sections 2-6 in use. The SL modular extensions is the option that full-time solar-panel cleaners we supply around Europe tend to go for due to rigidity and robustness in use.