Due to the usual tendency towards “software bloat”, Bryce 5 might run a bit faster.
Is there some compelling reason to use B7? Some new trick or ability?
If you seek “realistic” results then Bryce 7 is a better tool insofar as lighting is the most important part, and B7 has incredible lighting abilities. The good thing about B7 is that it still looks and behaves like B5. I find the Instancing in B7, while odd at first, has been a game changer allowing for the first time truly complex natural looking landscapes. Skies are more advanced in B7 allowing better sunlight, seperated cloud layers cumulus and stratus so different procedurals can be applied to each. Lights have both diffuse and specular intensity controls. The list goes on and on.
For me, there is no real need to go back to Bryce5. Bryce is generally far enough behind in features that even with the new stuff added since B5 its safe to say that B7 is far from bloated.