The van/cut-away chassis used under buses is not the same chassis as used by the vans of the same vintage.
While the regular vans were more of a unit construction without a frame the van/cut-away version had a rail suspension that was married to the unit construction front clip. Dodge pioneered the van/cut-away chassis in the '80's and later was copied by Ford and GM. The flat rail chassis made it much easier and less expensive for body upfitters to put bodies on the chassis.
Neither bus would be a first pick in my book.
The Ford has a better engine but the transmission was a very poor transmission that has not done well behind the diesel engines.
The GM has a better transmission but has an engine that has some known shortcomings and also faces the real problem of lack of parts support. When GM sold their medium duty truck line all of the parts and pieces for the 6.5L were part of the sale. As a consequence there is virtually no parts in any parts supply chain that will fit a 6.5L G-series van application.
This bus is in about the same condition and has about the same mileage but it has the 5.7L and a lower price: http://www.harlowsbussales.com/pre_o...p?veh=4484983\
This bus is a larger small bus with the 6.0L, a lift, and a lot lower mileage but it has a higher price: Chevrolet 2003 2003 Chevy Bluebird
This is another large small bus with high mileage but it does have the advantage of a flat floor: 1998 Ford/Collins – Stock #16UB037 | Western Bus Sales
Here is a bread box style bus on the P-30 chassis with relatively low miles: 1998 GMC/BLUE BIRD – Stock #16UB015 | Western Bus Sales
Personally I would stay away from the two you posted.
The P-30 chassis version of the 6.5L has the advantage of being pretty much identical to the 6.5L that was put in pickups, Suburbans, and Humvees. As a result there is quite a bit of aftermarket support for that version of the engine.
Good luck on finding a bus that will work best for you.