A mid-engined bus uses a standard truck chassis and is comparatively cheap to make. It also suffers from more NVH and possibly heat in the cabin.
A rear-engined bus isolates the passengers from NVH and also allows for clever things like a really low internal floor and wheelchair lifts/access. On the down-side, the control linkages to the engine are extremely long and may be exposed to damage by running under to cabin floor. The price is also higher.
In the UK Dennis specialise in rear-engined chassis for buses and fire appliances etc.
Don't forget the cooling problems that seem to haunt rear-engined buses, and the lube problems that go along with horizontal pistons (often used in a rear-engine app).
I have worked on both for years. The rear engine configuration is usually quite a bit easier to work on. As far as staying with truck components, this is also the case when the engine sits backwards in a longitudinal fashion. The sideways engines require the special transaxles and other components.
