I purchased a X738 three months ago (new), and had the exact same question you did. I spent numerous evenings worrying about the real world different between the 738 and the 758, and was totally in love with the 758 because the dealer had it in stock. In the end, I went with the 738 for two reasons: I have ethanol free fuel at a station near by, and I hate the smell of diesel (and so does my wife).
Did I make the right choice? No clue. I can tell you that my previous mower had a 24HP Briggs V engine, and it would burn about 3/4 gallon of gas per hour while mowing. With the 738, I can easily burn over 1 gallon an hour with the deck running (probably more like 1.25 gallons). I burn through a five gallon can each time I cut the three acres around my house, and it takes me a little over three hours (the previous mower took four hours). So, it sucks more fuel, but man it's fun to ride. With 35 hours on the 738, I've gone through about 28-30 gallons of gas.
In the end, with the average cost of ethanol free fuel ($3.99/gal here in MI), I think the diesel would have been a better purchase.....but with the 54" deck and 47" snow blower, but wife was already angry enough that it cost more than her car.
Are you going to be able to do anything with the 758 that you can't with the 738? No way. Will you save some $$$ long term even though the 758 initially costs more? Probably. If you're not afraid of doing the upkeep yourself and have a decent diesel source near by, I'd tend to lean that way.
FYI: When the dealer delivered my 738, we had a long discussion about fuel. I told him I run ethanol free (90/91 octane) in everything, and he loved that. I told him I might try running premium (93 octane) plus a fuel stabilizer/ethanol treatment, and he said "don't bother...stick with the ethanol free". I may still do it just for a comparison on fuel consumption.