One thing that the sedan will definitely benefit from (just as with the current Protege) is the racing effort in World Challenge... where the new 2.3L engine will prove to be a bonanza, with more displacement and variable valve timing... the sedans tend to be the pick for racing because of their increased rigidity (though I don't know what the relative difference is between the M3 4dr. and 5dr.) and, generally, lighter weight (believe it or not, it's not the weight of the stock car that I'm necessarily talking about, since these cars are gutted anyway, but the added structure necessary to make more rigid, the weight of the added glass (typical of Protege5 and the M3 5dr.) all add up to a 5dr. that doesn't get any favors, even stripped to it's bare racing essentials...
I'm getting a 5dr because more than just need the added utility(which I definitely do), I really appreciate the kind of flexibility that it allows me to do whatever I choose to reasonably do with a streetable car... and, I still like it more than the 4dr. (again, having seen both of them in the metal). It looks more aggressive, with the fender blisters (which also make it look a little less "thick" through the body section, especially through the front wheels) and which also make it look wider and lower...
If I was racing, I'd take the sedan but, for the minor penalty with the 5dr. (this is the street, after all), I'll gladly take the added utility and, to my eye, better looks. I'd guess that P5 owners made the same kind of judgement call, since the current sedan is lighter and more rigid there, also...