Getting sales data for games on the NES and SNES is a scetchy thing at best.  But most places will generally agree that Metroid, both as a series and each game indavidually, didn't sell as well on the NES and SNES as most of the other major core Nintendo titles (such as Mario, Zelda and yes, Kirby).  Even looking on this very site, it says Kirby's Adventure sold 1.75m units on the NES and Kirby's Super Star sold 1.44m on the SNES (along with about 3 other titles I'm not counting).  While there's no data for Metroid on the NES and Super Metroid sold 1.42m.  Then there's pretty much the absense of Metroid on the N64.

Then there's the whole problem of how Metroid sells like 90% of its games in the west.  Japan pretty much hates it.

So yes, you can say Metroid did pretty good considering.  But as for being more popular and doing better in sales, Kirby seems to do much better.  Note, what I said earlier I think is true.  I still don't think its at all a measure of Metroids 'quality' or 'third pillar' status.  Kirby just has more games and more general appeal.  Frankly saying Kirby is better because it sells better would be like saying Pokemon was better.

Starfox will aways have a special place in my heart:')

Same for me.That's why I want to mention Star Fox as often as possible.