Configuring Exchange 5.5 SP4

Having embarrassed myself yesterday by asking the wrong question, I ask again.

8 buildings spread across England, rolling out Exchange 5.5 and SP4. 2 meg links between buildings.

Should we go for Exchange configured for single site (MS seem to recommend and easier admin)? Or multiple sites (less traffic)? Views please, could do with early responses.

Jeff Fox
Service Manager
Highways Agency