I am about to purchase a new Zero Turn Mower. The possibilities are
between Kubota and Scagg. Can anyone advise or even suggest a better
brand and why? Do diesels perform better and I like the ability to sharpen blades often. Although Scagg is up on my list also.

Can't tell you about Kubota, but Scag is a pretty tough mower, cuts and stripes well. I've had a Scag in the past, but recently bought a guy out who had a new Gravely 260Z. It's been a real good mower for me so far. It will be hard for me to decide on my next mower between a Scag or Gravely.

Like any other equipment purchase.
Demo them all. If they don't offer a demo machine maybe they are hiding something.
Talk to other owners in your area. If the dealer won't tell you who the local owners are maybe you should look elsewhere.
Determine which local dealer has the best service and parts department. They all need service and repair at times.

Cost is seldom the driving force behind any purchase I make. Money spent well upfront will determine your eventual bottom line.

As far as diesel or gas goes, in most cases diesels will be more cost effective over the longhaul and will have better resale even if traded in every few years.

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Of course this is only my humble but well thought out opinion.
Bill
Northern Turf Tech
Kalkaska, Michigan