I value Gameplay first. If it's extremely good(and makes each one worth playing), then characterization follows next.
And they should go hand in hand too. They're both key to a game(that has storyline). Outside of perhaps Strategy RPG's, Racers, and Fighters, in other types of games, you want both so it's worth playing everyone. If they're all the same, there needs to be a reason. If it's just to hear that particular person's story instead of another, it's poorly implemented.
If everybody appears in the storyline, then the gameplay itself still needs to be good to be worth playing through the game.
I see them going hand in hand. Not one without the other.
I do not value them separately, because the first key to a game is having enjoyable gameplay. Since a character is there to play, they should all have good gameplay. Then there's the story. If it's also good, sweet. If it's not, the majorly good gameplay will save it anyway. But if only the story is good, and the gameplay is just terrible(or freakin' boring), most won't trudge through a boring game to get to good storyline. They'll all look it up on YouTube
, etc. Why? Because a key part to a game is playing it.
I want them all to play fairly differently so it's worth unlocking everything. If you can view all the cutscenes while only playing as the main, sure, fine. But if you need to go through with each one, and they're all barely different, then you're basically doing the same thing over and over again just to get another Final Story with Super Sonic, which... is just freakin' stale now.
The reason why Sonic Adventure 1 and 2 were so awesome is because no matter who you use, nobody is near identical to Sonic, or if they are(Shadow), the courses are still widely different(whether the same area, just plays different) so the experience is not the same. I didn't like the Advanced way of doing it because, well, there wasn't a huge amount of differences overall in the stages. It was the same overall stage, just slightly different routes.
I was not an actual fan of that style. Where in Sonic Adventure 1, 2, as well as somewhat in Heroes and 2006, all the courses were overall different for each set of characters. That means nobody went through the same thing in any way possible. The issue with the Advances is that everybody could go through Sonic route. The gameplay differences felt like they weren't capitalized enough. Where as in, say, Sonic Adventure 1, minus Tails since he sometimes does only an exact part of Sonic's levels, you were in the same "course", but it was actually entirely different overall. This is why I loved them. You never entered the real same course(outside of codes). Each level was entirely unique. Adventure 2 did the same thing, by being in the same place, but had an entirely different level. Mission Street was Radical Highway, etc. It was even neat using Codes to switch around what character appears where. And they could get eachother's upgrades, which was funny.
I thought both Adventures pulled it off perfectly. Heroes made it so each one still were widely different in the end. Sonic 2006 had no repeated levels like Adventure did, except it did it even better. Advances failed on that overall. I don't want the classic Sonic & Knuckles. I want the overall engine because it was perfect, but I don't want that style. I admit it did better than what Sonic 2 was, since Tails went through the exact same things, just had a few more options. Now, I know Sonic & Knuckles actually give the main two different levels overall. This is what I want. Actual different levels. No same levels, no near similar gameplay. Make each character unique. The cutscenes can be done regardless anyway.
I get why they should be separate, of course.(cutscenes and gameplay) But I don't like making only the cutscenes noteworthy and the gameplay of little importance. Ever.