Constant-Q is 1/3 octave constant width with symmetrical cut/boost, ‘american’ classic if you like.
Proportional-Q is based on accurate modelling of gain / Q relationship and band interaction in a British, industry standard GEQ. You get wide Q for gentle cut/boost, which progressively tightens beyond 1/3rd octave for higher cut/boost. Higher band interaction typically results in a smoother sum of adjacent bands when compared to Constant-Q (some describe it as warmer).
Hybrid is an asymmetrical model which gives you the best of both worlds: boost is proportional-Q, cut is clinical 1/3 octave.
Digi-Q offers minimal band interaction to provide a frequency response as close to the slider position as possible.
Most people use Constant for feedback control in monitor world, and Proportional for tone-shaping, but the choice of GEQ models depends on personal preferences rather than applications. Listen to each model and pick your favourite 
Hope this helps,
Nicola
A&H
