Hi Evan, The 17" is easier to use and better on the corners also riders better if the lawn
isn't perfectly flat also lighter for the wife to use
the 20" tends to dig in if the lawn isn't perfectly flat better for big lawns
I use a 20" on my lawn
cost is another factor
if you buy the twin rail model check that the rails are not cracked at the engine bolt holes
you can buy a good refurbished 17" for $650 20" for $800
if you want to talk I'm in Mandurah 0427449033
Cheers Ken
