Regarding to specs, recycletime for full charge is 1 vs 1.6 seconds, which is both pretty fast compared too many other strobes. And in real life you rarely blast off a full dump, so recycle will be instant with both strobes.
I can use my z240:s for more than 200 flashes without changing NiMh:s - I haven't tried more but it was a day with 4 dives.
Regarding power, I get the 125:s GN to 25 (air/meter) and the 240:s is 24. So that's not a real world difference.
/Correction; the 125s GN is 32 feet UW, which is about 21-22 air/m, so it seem it is actually slightly less powerful. Not a very big difference though./
In favour of the Z240 is that it's a lot smaller and nimbler. For me that was the most important factor as I travel with them. It also has a slightly wider beam angle.
/christian