The 1.2, 5 speed is most likely to be a wide ratio, which means it has the same 4 speed ratio's + another one on top for economy. I'd personally use that one out of the two you have, you'l most likely end up saving fuel as a result.
The best box for accelleration is the 5 speed close ratio, however means you'l be doing 4000 revs at 70mph. Not good for motorways!