I have used both, started off with the LAN8742 and had to go to the KSZ8091RN(A/D) when I couldn't find any of the old kind in stock. I just got my KSZ8091 running earlier today.
Other than the different footprints the changeover was pretty transparent to the code. I'm only using 'standard' register stuff and both chips were strapped to come up in auto-negotiation, I don't think I changed anything in my driver software to talk to them.
If you did want to try the vendor-specific stuff it should be trivial, you already have the ability to read/write to the PHY registers so the rest is just playing with bit flags.
I did find the documentation for the LAN8742 to be somewhat clearer and easier to read.