Maestro is more flexible and more scalable, so in most cases I would use Maestro. However, as @Maarten_Sjouw mentioned there are certain cases (especially in tele operator world) where a special hardware certification is needed. In those cases chassis is the only option.
1. Hardware
Depends on the number of appliances you have in Maestro if it takes more or less space than chassis, but chassis always takes the same number of rack-units.
2. Software
Not sure about the SSL inspection performance, but would assume Maestro is better as you can have more powerful appliances.
Clustering, VSLS and VSX are the same on both.
3. Administration is through the single management object on both and most CLI commands also work on both.