Different LED same wattage, what can I expect?

What kind of output can I expect if I drive a Luxeon I, Luxeon III, Luxeon IV, Nichia 1.5W all with just 1W? I'd be underdriving all except the Luxeon I but how different will the output be?

Well, in general, going from a Lux I at 300 or 350ma to a Lux III driven at 300 or 350 ma is it will probably be a bit brighter (depending on the bins of the Lux I and Lux III compared) and should certainly be more effecient.

The Lux V is a different design and in general, it tends to give more flood than the Lux I or III, and loses some throw. One isn't better than the other, but they are different. Of course, these are just general statements and this will be largely dependant on the reflector or optic used.

Never played with a Jupiter on it's own. Of the pics I've seen, they all seem to be quite blue... too blue for my tastes.

I don't know about the Jupiter, but among the Luxeons lumen output per watt is going to be pretty much the same. The biggest differences will be dictated by the brightness and forward voltage bins. I suspect there is no real difference between the LuxI and the LuxIII other than the current at which they are binned.

Wasblinded, McGizmo had a chart comparing different Luxeon and Luxeon-like LEDs at different current levels. It had S-binned Lux Is, T-binned Lux IIIs and U-binned Lux IIIs. At the lower current levels, an underdriven Lux III was definately more effecient. It can be seen here.

The S binned Lux1 beat the T binned Lux3 in efficiency at 350 mA, and both were beat by the U binned Lux3. Given the "width" of the bins and the rather small sample size, I don't think you can extrapolate the results to Luxeons at large.