I suppose that still leaves the question of how you can determine who "wins" a particular round of systems within a generation, huh? Well again, technical specs don't matter.
Marketing, perhaps? Hm, Genesis was marketed as heavily as (if not more heavily than) the SNES, and MS, Sony, and Nintendo all were quite heavy on marketing last generation, so... no.
First or second out the door? Let's see, SNES came out after Genesis, PS1 came out before all competitors that iteration, and PS2 came out after DreamCast. So no, that doesn't matter either.
How about initial game library? The SNES' library out the door was sizeable, but no match for the existing Genesis library at the time. And of course DreamCast had a much bigger library in place already when the PS2 launched. So it's not the launch library.
Lifetime game library? Well, yes, the lifetime libraries of systems which won their iterations were impressive, but much of that support came after the fact. So this isn't a fair criteria to judge by; developers will develop for the system they feel is going to "win", and there has to be a "winner" in their minds before that can happen.
System price? Well, the PS1 cost more than the N64, but there were a lot more PS1s sold. And the GameCube was cheaper than the PS2 for most of last generation, but PS2 still dominated. So no.
Brand loyalty? Ahahaha, no. If that were the determining factor, then Nintendo would never have lost support after the SNES.
Adoption rate? Well, there's definitely something there, but it's only part of the answer. The SNES, PS1, and PS2 all experienced faster adoption rates than their competitors. But why? Let's keep delving deeper...
Distribution base size? Ah, I think we may be onto something there! The SNES and Genesis were sold to largely the same markets, and ultimately ended up with close sales; where Nintendo dominated, Sega faltered, and vice versa. Ultimately, Nintendo's triumph over Sega was one of having a larger distribution base sooner in Japan and America. Sony released the PS1 in more countries than Nintendo did the N64. The PS2 similarly was launched in many territories as fast as possible. This model fits each iteration, and gives a definite answer: what determines if a system "wins" an iteration? Distribution base size. There could be more than that, but if there is, it's not part of the above.
Sky Render - Sanity is for the weak.
