I'm thinking the former, with the extra layer of plywood, for exactly the reason you mention. The extra layer of plywood wouldn't cost that much.

I agree. This way down the line if you ever wanted to do hardwood you would have the option to go any direction. Thats one perk but if you did decide to do any other type of finish flooring in there a reducer to the kitchen is easy and it pretty common to see different heights between rooms. Creates nice seperation.

Two for two, that's the way I was leaning as well. Mr. Weedy wanted to do it the quicker way, figuring we won't be around when/if someone wants to change it. But I'd rather spend $120 for the 6 sheets of plywood and have all options open for the future.

I agree also. Whenever I've run across additions which were made flush with an existing finish, I've always thought to myself, "Why did they do it that way?"

A subsequent homeowner would be happy with subfloors which are at the same level.