if you interested in seeing what the 8v can do and enjoy the feeling of gradually improving something then stick with the 8v and go for a bb head
if you want the quickest and possibly easiest route to more power and not that bothered whats done to obtain it , just put a 20xe in.
the xe head is pretty good to start with which is the whole point of it as both engines use the same bottom end.
so bb head just brings the 8v up to this standard. then both engines become fairly even after that power increases for both engines are pricey
,carbs / tb / turbo which is gonna cost loads which ever engine you start with so its really down to taste in the end.