Asynchronous or Synchronous Replication

Synchronous replication - guarantees "zero data loss" by the means of atomic write operation, i.e. write either completes on both sides or not at all. Write is not considered complete until acknowledgement by both local and remote storage.
Asynchronous replication - write is considered complete as soon as local storage acknowledges it. Remote storage is updated, but probably with a small lag. Performance is greatly increased, but in case of losing a local storage, the remote storage is not guaranteed to have the current copy of data and most recent data may be lost.

	Asynchronous Replication	Synchronous Replication
Data Loss	By its nature there may be some data loss [1]	Some solutions will guarantee no data loss [1]
Resilience	2 failures are required for there to be loss of service [2]  Failures which lead to data corruption will not be replicated to the second copy of the data [3]	A single failure could lead to the loss of the service [2] Failures which lead to data corruption are faithfully replicated to the second copy of the data [3]
Cost	Asynchronous replication solutions are generally more cost effective	Synchronous replication tends to be considerably more expensive to buy and manage than a comparable asynchronous solution [4]
Performance	Less dependent on very low latency, high bandwidth network links between units of storage	Dependent on very low latency, high bandwidth network links between units of storage [5]

http://sfdoccentral.symantec.com/sf/5.0/hpux/html/vvr_planning/pt_ch_planconfig_vvr10.html

Synchronous mode	Asynchronous mode
Need for Secondary to be up-to-date
Ensures that the Secondary is always current.  If the synchronous attribute is set to override, the Secondary is current, except in the case of a network outage.	Ensures that the Secondary reflects the state of the Primary at some point in time. However, the Secondary may not be current. The Primary may have committed transactions that have not been written to the Secondary.
Requirements for managing latency of data
Works best for low volume of writes.  Does not require latency protection (because the Secondary is always current).	Could result in data latency on the Secondary. You need to consider whether or not it is acceptable to lose committed transactions if a disaster strikes the Primary, and if so, how many.   VVR enables you to manage latency protection, by specifying how many outstanding writes are acceptable, and what action to take if that limit is exceeded.
Characteristics of your network: bandwidth, latency, reliability
Works best in high bandwidth/low latency situations. If the network cannot keep up, the application may be impacted.   Network capacity should meet or exceed the write rate of the application at all times.	Handles bursts of I/O or congestion on the network by using the SRL. This minimizes impact on application performance from network bandwidth fluctuations.   The average network bandwidth must be adequate for the average write rate of the application. Asynchronous replication does not compensate for a slow network.
Requirements for application performance, such as response time.
Has potential for greater impact on application performance because the I/O does not complete until the network acknowledgement is received from the Secondary.	Minimizes impact on application performance because the I/O completes without waiting for the network acknowledgment from the Secondary.