Hope I can help you with some of my experience:
Most Japanese (if not all) cars come basic option at Jebel Ali and all dealers have their assembly for upgrading whatever custom option you want including the leather seats. It may be a dealer or service adviser stunt to trick you to force you to upgrade to 3.5 for the wish of leather seats.
You can upgrade GENUINE real leather seats from best upholstery maker in Dubai / Sharjah for 3500 - 5000 AED MAX.
Difference between 2.5 and 3.5 is the instant pick acceleration difference and capability to quickly swerve between the lanes with that quick acceleration. If that's what your heart desire, close your eyes and pay extra - it's worth every dirham.
For long distance cruise as a daily drive 4 cyl (2.5) is far efficient than 6 cyl (3.5), factor 20% extra on the fuel with 3.5.
For long distance cruise as a daily drive you won't feel a jack difference in either.
So truly ask yourself what you really want, and do that. As decision made by heart will make you live happier than brain doing number crunching and forcing you to justify what make sense.