Here's the simplistic analysis...
The EFI will work better longer, but when it breaks you'll need a pro to fix it. (And so few mowers use EFI that finding the right pro could turn into a long-term search. So that question really depends on where you live.) If you are a pro and your fuel costs are a concern, then the EFI will cut your costs because they simply run at a more optimal mixture setting than the carb ever could.
On the other hand, the carb may have more issues over a set time period (say 6 years), but it's always stuff you can fix in your garage. If you'll buy a good fuel filter, a manual fuel shut-off valve, and install them in the fuel line right before the carb, and NEVER leave ethanol fuel in the fuel system over winter.... then 98% of your carb issues will never happen.
Hope this helps.
