Off the top of my head, I would say
you should switch to char. The advantages and disadvantages should be clear from what you posted already. If you are only storing a single chararacter, then the primitive char type takes up much less memory. On the other hand, the String class adds a lot of memory, and run-time, overhead. This cost buys the advantage that a String object is a bit more flexible. In particular, you can have Strings with as many characters as you wish, and different Strings can be different lengths.
I can only think of a couple of reasons to continue using the String class. First of all, if you forsee any of these fields possibly holding more than a single character some time in the future, then String will make it easy to do this. Second, JSP may have some differences, that I am unaware of, between char and String. However, I suspect that this later issue isn't nearly as important as the first.
In short, if you are sure that these fields will always be a single byte, my opinion is to use char instead of String.
HTH
Layne