Joanne, I have had several friends in interior design for many years now. There is LOTS more work than in product design WHEN the economy and especially the housing sector are rolling (like now); it’s very cyclical.
Good news for talented, experienced interior designers who tackle more than paint color and tile selection (i.e. do interior architecture almost) is that pretty much anyone on the street can be a potential customer, versus IndD where you can only work for an obviously far more limited proportion of the general population (manufacturing and related businesses) at any given time. Beauty with Interior is you deal always directly with the end user, making it an ideal field to start your own office on a low budget and make money sooner than designing mass-mfd. products for which you need countless suppliers and business relations, not to mention Olympian stamina and a very thick skin.
But don’t get me wrong, competition is fierce in InteriorD as well, you must love doing this above anything else. If you start out with doubts in your mind, others truly passionate about it will make sure you fail.
Financial reward should be your last criteria for studying ANY design-related field, it’s just the wrong way to start.
Talk to practicing designers in both fields, visit their studios to see what a typical workday is like and read up some books describing the business reality of each and the skills required.
Good luck to you.