It doesn't make a difference as far as forces go. They are exactly the
same on USB and serial. The only thing you get is a little bit less
system overhead (and, of course, the good feeling to use the latest and
greatest :-)
If you have it running fine on serial, then let it be that way. The next
system you have will have USB anyway, and then you'll be able to hook it
up there.
If you start getting other USB devices (maybe a mouse, or keyboard, or
speakers, or whatever gizmos you'd find), you can always upgrade at a
later date.
..tom