It's the same difference you'd find in any 50w vs 100w amp in my experience. I owned a Nitro with EL34s and also spent a couple of days with the Nitro SS. It's the same amp, same sound, but subtly fuller. I guess I'd have a slight preference for the Nitro SS because the low end is so massive in the 100w. I swapped EL34s with EL34B in my Nitro but thought it sounded best with plain EL34s. Some people swear the Nitro is best with KT88s but not my taste. If I had an SS now, I'd definitely put in KT77s though.
Take the JCM800 and Studio JCM 800 20w, or the JVM 50 vs 100w, Blackstar S1 50 or 100, or EVH Stealth 50 vs 100. They're both the same amps with a different power section. As a general rule, 100w versions of the same amps sound better at any volume level, so you often choose the 100w for home & stage. There is generally a certain fullness, smoothness, and lower mid presence in 100w vs 50w. You can usually dial in the 50w to sound 97% identical to the 100w with some relatively subtle variations in tone. In my experience, 50w versions are usually slightly raunchier and aggressive in comparison because they don't have those extra 2 power tubes to fill in a couple of peripheral areas of the EQ spectrum.
Note, however, that I'm generalizing for the sake of explanation. It's also possible to put Kt77s or 6550s in a 50w amp and it'll fill out like a 100w amp because of the inherently wider EQ girth of larger tubes. Obviously this doesn't always work. Some amps seem to like a certain bias range, so putting bigger tubes might sound like you just added a slight bit of low end to the inherent tone rather than filling in the overall EQ spectrum. My only experience with this has been the JVM 205 & 210 and Engl Artist 50 & 100. Where the JVM 205 still sounded slightly raunchier, though even better with KT77s, while the Artist 50 sounded like the 100 with KT77s. Keep in mind that I've been off Rig-Talk and out of the gear game for almost 4 years so it's not fresh in mind, and that's kind of my take-away from the experience.
There are major advantages to 50w amps as they're usually a little lighter, smaller, slightly cheaper, and cost less to retube. However, I find more often than not that 100w sound better at home so if your amps just sit in one place all the time, a 50w wouldn't really be and advantage if portability isn't an issue.
By the way, 6L6 amps sort of have more diverse tube options than EL34 (if your amp can fit taller|wider tubes). Options in EL34 are basically EL34B, EL34L, and KT77. The EL34 B & L are EL34s with a difference in the low end: bassier or tighter, respectively, but still sound just like EL34. The most different sounding but totally compatible is KT77.
Now 6L6 has a range but not quite as compatible as EL34 types. You can swap 6L6 with 6L6GC, 5881, 6550, KT66, and sometimes KT88. Now, I'm just a guitarist so I don't know the technical aspects but it kind of comes down to if an amp's sockets and transformer will handle bigger types. Maybe someone who knows what they're talking about can clarify this. Going from 6L6 to 6L6GC is most similar. 5881s have a different feel, compression, distortion. 6550 might sound like a broader, more refined 5881 perhaps. KT66 and KT88 have sort of a more 3D sound, very clear, and full bottom end - especially KT88. 6550 might have a bit of that 3D quality depending on the brand, so JJ 6550s might be more like 6L6/5581 but Tung-Sol might sound a bit like KT66. KEEP IN MIND: This is how I remember them and experienced them, and I'm only capable of very basic amp biasing like JCM800 or amps with external bias.
Anyway, I just explain because tube substitution is probably the best mod if you have a lower watt version of a bigger amp. Well, I hope this helps.