i think i would prefer the industry without sony in it to be honest with you. i love the playstation, and to a lesser extent the playstation 2, but at this point, the ps3 and ps4 have been incredibly useless, participating in this competition with microsoft that has done nothing but dragged this industry through the mud. they have absolutely no respect for what gaming is.
if it were just nintendo and sega, the differences that i see are 1) gaming would have stayed niche for longer. at this point, it would be a lot more mainstream. 2) online gaming wouldnt be what it is today (which is probably a good thing, guys. think about it) and 3) games would be more evolved than they are now. 90% of the games that are released would focus on the important aspects of gaming rather than cutscenes and turning them into movies