I may be bias but I happen to belive that both the mainframe and the IBM i are both going to be needing programmers well into the next 50 years. There is already a shortage of trained staff due to the dot com bust and the outsourcing of work to India which companies are finding out does not work. It really depends on what size company you want to work for because mainframes are in very large companies while IBM i is in the smallest shops to the largest shops. Both are business machines that run the worlds banks, manufacturing companies, etc.
Personnally I like the IBM i the best because it is still based upon far more integrated and more reliable propriety operating systems so things always work together. The underlying single level storage and virus proof capability of each software object is predefined like files, programs, user profiles, subsystems, etc that never can change its type like the unix and windows environment can, makes this platform one that will continue to be bleeding edge for years.