What are the advantages and disadvantages of riding a smaller Mountain Bike frame?
With a smaller frame, you will need an appropriately longer handlebar stem in order to retain the cockpit length. This will affect handling of the bike: it will feel more "lively", which can be good in some cases (tight turns, difficult terrain) or bad (less stable) - in most cases.
A smaller frame will be marginally less reliable, and more prone to break, due to bigger stresses in its structure; longer stem and seat post mean bigger levers to exert force on the frame. But this should not be a significant effect.


As a rule of thumb, a 17" frame is an inch or two too small for your size. I would not recommend taking the smaller one without actually riding it for a couple of hours and making sure it's comfortable for you.