I have 1.7's. I feel they are the sweet spot for quality/value to price. The 3.7's are a step up in performance and price however for me it's not worth it. The 20's are a different story though but very much higher in price.
Using an active crossover really changed the game for the 1.7's. Setting the low pass filter at 70Hz and 18db/oct opened up the higher volume listening quite a bit.