With government planning the way it is, diesel will be phased out either by further legislation or simply taxation. I’ve noticed that the price gap between petrol and diesel is mounting, 20p+ around here, which means you need to get a good chunk of extra miles per gallon before petrol, or more accurately hybrid, actually becomes cheaper to run.
To answer your question. Extending will keep you in a Tucson for that smidge longer but I can’t see the scheme being any better for you next year or any year after. In fact, I’d suspect that your options will become hybrid, petrol or none within the next 5 years. If you can afford the outlay I’d be test driving both the Merc and Audi and would put an order in, in anticipation of the car arriving soon after the lease expires on the current Tucson.
I'm Autistic, if I say something you find offensive, please let me know, I can guarantee it was unintentional.
I'll try to give my honest opinion but am always open to learning.
Mark