Your 20mm will effectively be a 10mm when you add the barlow. So you might want to think of purchasing one of a different FL so you get good value for your money.
Personally, my 15mm SV gets the most use. Followed by my 2" 28mm.
I use the 40mm for tracking down comets mainly.
My last scope came with a 9mm and a 25mm. Both have 52 degree fov, and quite nice considering they are just kit lenses.