It would have to be meaningful. I wouldn't want it done in a big public place like a restaurant or - God forbid - a football game like that old cliche, it would be just the two of us in a romantic moment. I don't care if it's spontaneous or planned, as long as it's a sweet or emotional moment. In fact I might prefer it to be spontaneous, because a lot of the time all the planning that goes into the moment just adds pretension and all the bells and whistles become more important than the love and happiness of the moment. It'd be more about the show of it all than about us.
One thing I would not want though, is for him to get down on one knee. I would not do it for him were I proposing and I wouldn't want him to do it for me. It's degrading and it just looks silly. We'd have to be at eye level or something, so one of us isn't looking down on the other.
If I were not ready, how they propose wouldn't matter. I would still say no, but make the reason clear and make sure that it wouldn't colour the future of the relationship. Also, I think if I rejected them then I might have to propose next time. It'd take the pressure out of the moment because I'd already know he's ready since he asked me already :P