As far as I'm concerned, any 400 yard par 4 is a hard hole. So I'd say two 400s are more difficult than a 350 + 450.
Beyond that, I'd say the 350/450 combo would be more *fun*, which I think is much more important. They provide variety, the shorter 4 is a better scoring opportunity than any 400, the 450 is hopefully an easy 5 - and it'll be listed as a par 5 for me anyways, so it's a good scoring opportunity too. (I'd even prefer a 300/400 setup to a 350/350 setup, as far as that goes.)
I also think the hypothetical bogey golfer would prefer the 350/450 as well. I bet he'd get plenty of birdie opportunities on the 350, and only rarely on the 400s. Who doesn't want more birdie opportunities?