A synchronous HO for colocated sectors is quicker since you do not have to send the “Physical info” across (no need to measure TA, acquire synchronization). So as a general rule, better to define colocated / co timing source cells (whether same BTS or 2 synced BTS’s) as synchronous.
Asynchronous will work too but why spend more time unnecessarily.
For 2 non colocated cells, defining neighbors as synchronous could be disastrous, unless special set up in special cases like HO on railways where u can have a prdetermined TA for HO.