Novas,
Visiting a unit before stage 2 / 3 doesn't necessarily give you an advantage on the day, but it may show aptitude.
More importantly though you get to meet people who do the job for a living and can tell you what it's really like (depending what you ask
). If you are committing your career to the job, best to go in eyes open.
If you can, get to a centre and a tower to see both disciplines, after all, you could end up posted to either, so you need to be sure you'll be comfortable with either.
Either way, good luck.