Ah, I see another mistake that I made.

Megabus clearly identifies the route as being operated by SWT, while the other search I was doing was on the Southern Railways site. I didn't notice the obvious difference (different names, entirely different logos). So when I went looking for pics of the carriages, I ended up only looking at the one for Southern.

Fares are about the same with an advance purchase (£4), both stations seem easy enough to get to. But it sure looks like the seats on the 450 are a lot narrower than on the 377 (I see that they fit 5 across).

Thanks all for the tips on storing luggage between the seats instead of in the upper racks.