Main office and remote Office Exchange Setup

I'm interested in peoples views on if there is a standard setup for my particular scenario or any methods you could recommend.

High level overview is a Main office and a branch office, around 30 people in each. Currently 1 exchange 2007 server at the main office and a 2003 server at the remote office - 2 different domains and namespace. Plan to bring these together under one domain with VPN links for the  2 sites.

Whats the recommendation for Exchange - 2 exchange servers with a mailbox role at the branch office or just one exchange server at the main office accessed over the VPN.

I would be interested to hear peoples opinions.

Thanks