I don't ever use my laptop laying down, so can't speak to that (kind of hard to edit photos or type documents laying down), but I do occasionally use my 13 inch on my lap. Honestly it's a toss up, I used to have a 15", and I can say that I had more concerns with it sliding off my lap than the 13". But the difference isn't noticeable.
If it were me though, if you plan on using it on your lap/bed a lot, my suggestion is to get an iPad or something of the sort for use in the bedroom, and if you need a MacBook, leave that at your desk. MacBook Pros work better on a flat, non-insulating surface, and besides that. Two different devices for two different needs.