Actually it is the same, if you do FV50, the next step to clear line items (matching between Dr. and Cr.) will be F-51. But if you do F-53 the 2 transactions will be happened (merged) at the same time. I just know this once Fon advised when she join us.
Remark: F-53 can do only for some account code which we call recon. account. if not it cannot be done. So, you are better check with Fon/K.Hong if it is applicable for the transaction that you would like to key.