Last month rent vs security deposit. Which is better

[John Gross]

Hi Everybody, I am new to bigger pockets and long term leases. I have a young couple wanting to rent my adu at my house for one year in Arkansas. 

I see on the lease agreement an opportunity to charge last month rent and or security deposit. Is one better than the other? Why? I don't want to overwhelm this young couple with too many charges so trying to figure out best solution. 

Your feedback is much appreciated!