Permanent ramps are structured of concrete, such as the ramp shown below and is ideal wherever regular wheelchair access is required.
With a non-slip surface, side upstands and easy grip handrails, it is equally suitable for ambulant users with reduced mobility.
Pros to a permanent ramp:
Cons to a permanent ramp:

Semi permanent ramps are constructed from mild steel angle and tube with a raised mesh walkway surface galvanized after manufacture and comes with adjustable legs, levelling points and handrails.
Pros to a semi permanent ramp:
Cons to a semi permanent ramp:

What length ramp do you need?
For fixed concrete ramps, Building Regulations would generally suggest a gradient of 1 in 12 (e.g.: a 3″ height would require a 36″ ramp).
However, this is a guideline, and not a legal requirement. If space permits, this is the ideal gradient, but often the height that needs to be bridged is too great to allow such a length, particularly on narrow footpaths etc.
Where only occasional use of a portable ramp is required, for powered scooters / electric wheelchairs (or a manual wheelchair with assistance), a gradient of 1 in 6 is adequate. (e.g.: a 6″ step requires a 36″ ramp).