conceptual multisite domain question

Client has offices in NY, LA, London, and Florida(1 user).  LA office is upgrading their servers (new hardware, W2k3), and don't mind starting from scratch.  NY is runing Native 2000.  IPSEC VPN has been established with and connectivity is possible.  T1 connection on both sides.  50 users in NY, 20 users in LA, both offices could double in next 3 years... but may not.

dns for mydomain.com is handled by ISP and www.mydomain.com is hosted.  internal ny domain is ny.mydomain.com.  Up until this point la's domain was mydomain-la.com.  We would like to consolidate the domains.

do you think it's better for LA to set themselves up as la.mydomain.com, and have trusts between the two offices?  or have them just join the ny.mydomain.com.  I would rather have them join, because there is no network admin at LA and would like to centralize administration.  I just don't like that LA office will be part of "ny".mydomain.com.

Exchange will be running in both places.  Currently MX records for mydomain.com points to ny office exchange server, and MX record for mydomain-la.com points to la office exchange server.  we are planning on only using mydomain.com for email, and pointing the mydomain-la.com MX record to the ny office (so la won't lose email addressed to their old addresses), so that it will handle mail to both domains, and route to LA based on destination mailbox.