The filter sizes are different. My other lens is the 70-300 IS USM with a 67mm size. To date, Ive been using two different sized polarizing filters for the 70-300 & 24-105. I have anticipated using the SR variable ND mostly on the 24-105, but if I had the 24-70, I don't know yet like you said if I might find myself using that one more than the 105 for landscapes.

The main reason for the 24-70 purchase is pet photography (faster lens) and low light/indoors (events). Thought I could get away with the 24-105, but as I use it more, Im running up to the limitations of that lens not being fast enough.

Was hoping someone else went through this before and could part some insight.

Im not in a rush to purchase the filter... Got christmas money as a gift and wanted to use it towards the filter. I could save it until I get the 24-70 later in 2014 but then I won't have the filter for the spring waterfalls Im hoping to use the filter for.