It depends on your market, here are some points to consider:
1. If your home is currently a two bedroom, adding a bedroom is probably the best bet.
2. If it is a three bedroom and one bath, I'd add another bath.
3. If it is a small three bed, two bath look at adding a family room.
In general the cost of construction is less than purchase. For example, according to Trulia the average price per square foot is about $200. So if you could build a bedroom for $100 per square foot, you'd be "making' $100 profit per square foot.
Talking with a Realtor that knows the local market would be a great way to start.You want to avoid over-building.
Just one quick thought, too: $50,000 to remodel a master bath is a lot in most communities. Again, I don't know your house or area at all, but $25,000 is a lot more in the range I'd expect...perhaps less. And recognize that while remodeling the master bath will certainly make your house more saleable, it won't return anywhere near 100% of your investment.
And if you're not planning on selling right away, you have to factor in your own preferences. Which do you need and want more? Which is of more value to you? Even though, as I said, a major remodel of the master bath won't return your investment, still--if that's what you want and what you'll enjoy--then that's what you should do.
Hope that helps.