If you haven't been to the Fens it might be worth swinging north just for the experience. But I wouldn't choose it for interest, it is an empty landscape and there is basically nothing there. Though, as has been pointed out upthread, Ely cathedral is lovely. Of the two options, I would choose the southern route (but I have been to these places before).
However if you change your midpoint from Diss, then even more interesting options are possible. If, for example, you were to stop at Stowmarket (which would be about the same distance), then I would go Cambridge - Clare - Lavenhem - Stowmarket - Debenham - Halesworth - Lowestoft, which would be a much prettier and more interesting route. It must be said that Stowmarket isn't very exciting, but then neither is Diss. If you want a nice town to stop in then I would choose Bury, but this might throw your distances out.