Shredded Wheat is 'better' in the sense that it has no added sugar or salt, but weetabix is by no means bad for you (until you dump a load of sugar on top).
I love them all but porridge is the best for you hands down.... Its in every healthy eating magazine and that genius nutritionist on tv, Gillian McKeith, recommends it, although, as stated above too much of anything isn't the best thing to do.
Also, an elderly gentleman at work has lowered his cholesterol considerably after his nutritionist recommend he eat porridge. X