I was lucky to own an Einstein top, where I could pull two tubes to compare the two options. 50W was kind of "further away" sounding, a tad more ambient, less "fast" and less headroom, so when you hit those strings, the 100W were more dynamic, even at low volume.
I preferred 100W at that occasion.
It's also a matter of design I suppose, e.g. my Shiva is 2xEL34 only, but still very tight.