I am not sure 50i should be compared to 35d. Someone who buys 50i usually doesn't put mpg on top of priorities.

I have 35d, but would go for 50i

Driving 60-65 on highway, you can crack 600 miles on one tank. I have done that once, but it takes some discipline.

Most long trips here in wa are driven at 70-80mph.
There's something about 35d driving at 80mph. The RPM's are in sweet spot, where pressing a gas (shall say diesel) pedal results in a quick response and vehicle accelerating without downshifting.

If you have a choice, go for 50i. If it's above your budget, make a choice between 35i and 35d.