buy the biggest one u can afford and just use the height u want, i wish i had done his as i didnt realsie how easy they came apart i have an xtel pole 24ft
If you are going for just one pole then I agree and that is how I started when I bought my first SLX in March 2009 - I got an SLX35 and used the top four sections (20ft) for most of my work and added the others when necessary.
I have now (delivered yesterday) got an SLX22 for all my domestic work (which I'm gonna try at 18ft by taking off the outer section as it should do virtually all the stuff my current 20ft does) and have the SLX35 ready for the higher stuff.
If I was buying my first (and only) SLX pole now I would go for the highest telescopic I could afford and just use the top 18/22ft most of the time.