They are two different species in the same genus. Basically, it's like the difference between musa basjoo and musa dwarf cavendish. They're in the same general group (ensete) but they are different species.
Ensete ventricosum is typically a green plant but sometimes has red stripes on the petioles/midribs. It looks like this:
Also contained within the E. ventricosum species is the E. ventricosum 'maurelii.' This is a ventricosum with redish cololoration on the leaves. It is a common landscaping plant found at places like Home Depot/Lowes, etc. It looks like this:
Ensete superbum has green leaves and with time develops a very large, fat, brown stem from the remnants of old leaf bases. It looks like this:
Both produced seeded/inedible fruit. It should also be noted that E. ventricosum is fairly common (especially the 'maurelii' subspecies). E. superbum is much harder to find and the seeds are quite difficult to germinate. E. ventricosum seeds seem to be much easier to germinate.
Hope that helps!