You're comparing someone who's clearly seeking fame to someone who isn't. Do you think that if an artist like Gaga (or any pop star really) had an album like
21 they wouldn't milk it for all it's worth? They'd be at their 7th/8th single by now, all of which would be accompanied by a massive music video, and working on a re-release.
Adele gave us sitting in a chair, walking, a live performance and the
possibility of a 4th video. (I actually love her videos for RITD and SLY). She said she's here for the long run and that she was ecstatic about
19's success. So as long as she sells couple of millions WW/per album she'd be happy (and everything she's done so far shows that she was telling the truth).
As for Gaga, it'll depend on whether she will go down the 50-cent road (major benefactor of a specific genre's peak who dominated for several years
) or the Eminem road (respected artist who still sells to this day regardless of his genre's popularity). Her fame could easily disappear once electro-pop/dance fades unless she reinvents herself. The next couple of years should be very interesting for her.
I think both could have great longevity and be remembered...but of course the OP didn't give us that option
If she's one of the best vocalist of all time, means she's better than Adele right in technical singing skill and has a stronger voice right? If the reason why people bought 21 is because of Adele's voice, why can't we use the same theory to Christina? I mean, why would one of the best voices can't sell yet still working as hard as ever? Ridiculous. I guess there's more than just voice and image then. 
