Howdy,
The short answer since I've got to dash to work early today...
The R-Zilla Tropical 25 probably outputs half of the level as the Tropical 50. Since the Tropical 50 was able to actually kill chameleons in less than a couple of days, I'd say that the tropical 25 would take twice as long to kill a chameleon

. I was involved with the initial investigation as to why chameleons were being sunburned alive by the "50" and helped pay for some of the analysis. A loooong story short, both the 25 and the 50 have shifted spectrums and output far too much energy. If your chameleon happened to climb on the screen right under the tube's surface it would receive something like 1600uW/cm2 or about 100x what it should get. I have photos of chameleons that died from exposure that created blisters etc. The manufacturer pulled the 50 and is re-formulating its phosphors. For now, don't buy either until the new phosphors are evaluated.
It's all written-up here:
http://www.uvguide.co.uk/phototherapyphosphor.htm