The destination disk type is about how an ESX will handle disk, the thick disk will reserve all disk capacity while the thin disk will grow if necessary.
You can predict your transfer based on used space on disk, however the speed of transfer depends on many different factors and can not be calculated precisely.
From the other hand there is an incremental synchronization which will be useful in your case - the down time is minimized because initial sync will transfer 'base' data and incremental sync(s) will transffer only differences.
HTH