Firstly I do not think anyone will you which way your career should go. This is your choice and yours alone. This means that anything that happens in your own life is your own fault GOOD or BAD.
What you should look into is:
1: Will the AS400 (System -i) be around while I am climbing the corporate ladder.
2: Is there expansion in product to allow for product growth…. ie keep you in a job till you retire.
3: Look at recruitment sites to see what AS400 jobs are out there. If there are loads this may meanit is a good choise.
4: Speak to people doing the work and see what they think.
There are allot more to think of but I hope this makes sense.