Actually Zaphod does have 2 heads in the movie too, just not on screen all the time, the other was hidden.
I can actually understand why they didnt give Zaphod two heads right along side of another, its a tough effect to pull off even with todays CGI and the prop in the BBC series is perhaps the worst part of watching it.
As for Marvin, yeh Marvin was not as good in the movie.
However I did like Mos Def as Ford, in truth I think he did the role better then David Dixon in the BBC series...
The part where he tries to speak car is hysterical
