That would really depend on how much effort is lavished on them. I don't think the Dreamcast > Xbox idea noted in this thread is a million miles away from what we have. I'd need certain specifics that I'm not privy to in order to know whether it's a bigger, smaller, or possibly closer to a PS2 > Xbox sized gap. It's never required an unbreeakable grip on reality to know that it's nothing like the crazy gap seen this time around.
In real terms, a lot rests on the market; how these two are embraced (or not) and how high Nintendo hitches its skirt. Xbox was seldom really exploited, because there was no reason for it to be the main concern. If Sega decided to downport, say, PDO, there'd have been more than a little bit of corner cutting in order...