Sign lease or sign deposit to hold agreement

[Walt Dockery]

I apologize if this has been asked and answered already.  I searched and didn't find a response to this particular question.

I have an existing tenant who's lease expires July 31, and she will be moving out.  I have recently begun marketing property with availability August 1.  Funds to move in are first and last months rent plus a security deposit.  My question is, if I line up a qualified tenant now (or soon):

Do I have them sign a lease now (for a lease term obviously beginning Aug 1) 

or 

Do we sign a deposit to hold agreement now, then sign the lease Aug 1 when they move in, hand over keys, and provide first and last month rent via certified funds.

In either case I plan to take a deposit to hold now that would roll into their security deposit.

Obviously the above assumes their application, credit check etc. all checks out.