I have 4 acres that is rolling and some rough areas to mow over, I cleared half the land from shrubs and debri from being unmowed for years by the previous home owner. Looking at the mowers I dont see much difference between this and the 540m. I really don't want to spend the extra to get a to the next series which is semi-commercial according to the sales rep. What is the difference between the 525/540. Is it too late to upgrade? It hasn't been delivered yet.