The light output of the latest generation lamps (zenon) is claimed to be 30% higher than Halogen.
The problem is that the eye's perception of brightness is logarithmic, so that 30% more amounts to "not much".
Having seen them I think that's about right.
To get a really noticable improvement you have to go to HID, but you're talking several hundred pounds for the conversion because of all the control electronics. These prices are sure to come down, but not for a few years........
So I guess the answer is: if you need a new bulb, go for a zenon one, because they are a bit brighter and don't cost much more, but the improvement is not worth changing a working bulb for.
'73 Jensen Interceptor (Resting)
Giant Full Sus Mountain Bike