I planted with SF1 for a few years and got along fine with the occasional shift.  When I put row clutches on I started using SF2.  It's spendy, even for just 3 months I think it was like $600+.  I planted the first field before I realized I had not activated the SF2.  I could tell when the corn came up that I was using SF1.  You could see the guess rows from time to time and the clutches weren't perfect towards the end of the field.  I feel SF2 is worth it if you have clutches.