After doing a lot of ring and cog switching on my previous bike, I came to the conclusion that I spend most of my time in the middle gears so there is where I wanted everything to work the best. What I am saying is that expanding your gear range will cause the middle of the range to suffer. 90% of my riding is between 8-12 MPH.

My suggestion is to go with a 34T cassette first. Try it fully loaded, then if you feel that it needs lower, go with a 24T ring. Your wallet will thank you for not changing to 10-speed.