I use the slx18 for 97% for my work and still don't fully extend for a large percentage of that, I also have a Slx22 and Slx25 really dont like Slx25 as I find it bendy compared to the Slx22 with an extension.
if funds are available try and get a slx18 as it's really light work and another pole like the Slx22 or Slx25 for the other windows that require a longer pole. Also I have a few really well paid jobs that require a couple of extensions for the odd velux window 3rd floor windows etc the extensions takes seconds to had on and paid for them self many times over and because they hardly get used probably last me a life time.
if funds are available try and get a slx18 as it's really light work and another pole like the Slx22 or Slx25 for the other windows that require a longer pole. Also I have a few really well paid jobs that require a couple of extensions for the odd velux window 3rd floor windows etc the extensions takes seconds to had on and paid for them self many times over and because they hardly get used probably last me a life time.