Another viewing before we sign the lease?

you stand to forfeit your deposit and might still owe the broker a commission on deal #1

Rude ... maybe
Lose your deposit ... probably

Same broker? First of all its not rude at all. Second, if it's the same broker and he or she is any good if you change your mind from one to the other the broker should be able to finesse with the landlord. Unless that landlord is a jerk, like rb345

It's not rude at all however there's always going to be another apartment coming up. At the point you put a deposit down you should really try and stop shopping around. If it's the same broker and building you can likely get the deposit transferred to the other apartment. Typically brokers hold the deposits not the landlords so you should be able to get at least part of it back.