I'm wrestling with that same idea on mowers what good is having a great motor when everything else is worn out. I think it comes down to price I personally wouldnt pay the extra to move up from say a 27 hp gas to a diesel but, when you start hitting 14000-15000 per mower on the bigger motors 31-34 the prices are very similar then. So I think I might just bite then. But it comes with other issues like you can no longer steal the mowers extra gas you have in the can to make 2 cycle.
If you are doing this fulltime, I would say go with diesel. More torque, better fuel economy(cheaper per gal. too). If your really concerned with production, your not going to have to slow down much in high or wet grass and more than likely only have to fuel up one time. I would look at the kubota zd mowers, theres a couple of guys around here that use them with the new zd pro deck and they leave an awesome cut quality and they cost approx. 9k-9.5k. not much more than some gas models.