It depends on a couple of things. The most important element is: What would buyers expect or want in a home similar to yours.
Specifically, in some cases, they'd want that extra bath. In other cases, they'd want that extra bedroom. And that's something that you'd need a local Realtor to advise you on.
Then, since you say you plan to stay in your home for at least another 5 years, the equation tilts a lot more to what would make the home liveable for you, regardless of return on investment.
A couple of considerations:
In many areas of the country (but I'm not speaking for your area), that extra bath would probably be a smart project.
On the other hand, now you're separating the bedrooms--two up and one down. And many families with small children wouldn't like that. So you'd be narrowing your market when it came time to sell.
Another question: While you'd be constructing something in the basement, could it legally be called a bedroom? It depends on your local regulations, but often a "bedroom" requires both a closet and some sort of exit--a walk-out, for instance, or a large-enough window. So, functionally, you could use it as a bedroom, but when it came time to list the property you might have to call it a 2 [legal] bedroom/2.5 bath. That probably wouldn't be a good idea.
Another possibility which you may already have considered: Remodel the basement and add a half bath down there. Right now, it sounds as if you don't have a bath down there. People will appreciate the convenience of a bath (even a half-bath) down in the basement. Fix the basement up, brighten it up, and add a new half or full bath down there. That way, you won't be separating the bedrooms. The trade-off is that you won't get a master suite.
So: Consider those possibilities. Check with a Realtor to find out what buyers want and expect with your type of home. But then--really--make the decision based largely on your needs.