DAG across 2 Data Centers

Looking for pros and cons of 2 potential Exchange 2013 implementations.
ADSite1: 400 users
ADSite2: 100 users
ADSite3: 50 users
Implementation 1: (DAG across two Data Centers without DAC implemented)
ADSite1: ExchSrv1 (MBX/CAS) --- DAG (2 DB) --- ADSite2: ExchSrv2 (MBX/CAS)
ADSite3: FSW
Implementation 2: (DAG across two Data Centers with DAC implemented)
ADSite1: ExchSrv1 (MBX/CAS) ExchSrv2 (MBX/CAS)--- DAG (2 DB) --- ADSite2: ExchSrv3 (MBX/CAS)
Site3: FSW
1. Am I gaining any true benefit from Implementation 2 (additional server in Primary Site)? i.e. Implementation 1 covers me for HA and DR. Would it make sense to consolidate ADSite1 and ADsite2 into a single ADSite for Implementation 1?
2. In either case, is it ok the configure NLB for all the servers (for the CAS role). So, if a user on ADSite2 hits the CAS on ADSite1 they could then be proxied to either ADSite ExchSrv depending on where their MBX is.
3. If all the MBX DBs in ADSite2 are replicas and not active and a user hits the CAS on ADSite2, is this increasing network traffic to then allow ExchSrv3 (CAS) to have to proxy to the ExchSrv1 (MBX). If so, does it not make sense to have the ADSite2
server only hosting replicas?

Hello,
1. I recommend you use Implementation 2. When you enable DAC mode, it will prevent split brain from occurring by including a protocol called Datacenter Activation Coordination Protocol (DACP). After a catastrophic failure, when the DAG recovers, it won't
automatically mount databases even though the DAG has a quorum. Instead DACP is used to determine the current state of the DAG and whether Active Manager should attempt to mount the databases.
2. You can deploy CAS NLB, but you can't depoly DAG+WNLB. If you deploy NLB, the CAS will proxy traffic to the Mailbox servers hosting the active copies.
3. If you deploy NLB, the ExchSrv3 (CAS) may proxy requets to the ExchSrv1 (MBX).
Additional article for your reference.
http://blogs.technet.com/b/exchange/archive/2013/01/25/exchange-2013-client-access-server-role.aspx
Cara Chen
TechNet Community Support

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From your description, I would like to clarify the following things:
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Planning for High Availability and Site Resilience
http://technet.microsoft.com/en-us/library/dd638104(v=exchg.150).aspx
Exchange 2010: Collapsing DAG Networks
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