No, SA-X could destroy a planet, but Dark Samus is stronger. Its weird Fusion and Echoes are too much alike, well there alike in a sense. Both have an enemy that resemble samus in form, both have a new speices that can copy creatures and, well thats it, really, but still those are two important aspects.
Isn’t it obvious…whatever I post, I am unsure if one has already said…:
DarkSamus would totally win…we all know that Samus rules, so even though the SA - X is basically Samus, you can still beat the SA - X! same with Dark Samus, but just look, man! Dark Samus, not only looks cool, but can also like do COOL stuff…ehhhhh?
just because something looks cooler doesnt mean its stronger. the reason DarkSamus would win is because DarkSamus can only be harmed by phazon, and the SA-X has no Phazon abilites.
if im wrong correct me, i havent beaten metroid prime 2 echoes yet cuz im to lazy to, but if dark samus can only be hurt by phazon y do we see her absorbing it so many times in cutscenes and stuff? she was even born from phazon. 
But what if Sa-x multiplies into more SA-X and they all fight against DS they may have a little chance I think… 
to answer your question,Perfect_Chaos9 Its sort of like Samus with the Gravity suit on her in the 2d ones. Samus can not be harmed by lava and heat but she can still be harmed by heat-based attacks, and DarkSamus cannot be harmed by Phazon, because she is absorbing it to her self it is not harming her outer suit, I think, or its something like that, Perfect_Chaos9, dont read this…
Perfect_Chaos_9, it’s because the only way to penetrate the shielding that Dark Samus puts up is through a beam of Phazon. The Phazon isn’t what hurts her, it’s the energy in it.
Think of it this way: The Phazon Beam uses two things: energy and Phazon. The Phazon provides the means of penetration through the shield, and the energy is what harms her.
It might also be related to causing an overload of Phazon, but I doubt that.
In any case, the bottom line is that it’s not the Phazon itself that harms her, it’s the combination.
Anyways, about the SA-X vs DS:
This is by far the best point i’ve seen in this topic. The SA-X reproduces by asexual division, and if it did that enough it could overwhelm DS with clones. However, what we’re talking about is a SINGLE SA-X versus DS, I think. Still, it’s something to note: The SA-X, in a no-rules, unregulated conflict, might just win by overwhelming DS with firepower.
I just want to point out, in addition to that, that in the aforementioned battle, the SA-X can absorb memories of it’s prey. This means it could access Samus’s memories and remember what beat DS: The Phazon Beam. It could then quite conceivably develop a method to absorb Phazon somehow and use the Phazon Beam on DS, if it could find Phazon. Ten SA-X’s + Phazon Beams + DS = Dead DS.
However, in a single, one-on-one, no-extra-abilites battle, Dark Samus would win. DS has a total tactical advantage: she has all of Samus abilites, but better, can fly, can put up a nearly impenetrable shield, can reform herself short of complete atomic disruption, and go invisble. The SA-X can fire the ice beam, use a power bomb, or fire a couple of missiles, with some other misscelaneous abilities. By that alone, DS would win.
Uh, your first point… “It could be a phazon overload but I doubt it”? That’s exactly what happens. It says so ingame. >_> The scan data for DS4 states that she absorbed too much phazon and that it made her unstable. You attack her with more phazon. Obviously, the phazon you attack her with just makes her even more unstable. 
As for the SA-X multiplying… it would take them time to regrow into a full SA-X, right? I mean, it’s not like she can just instantaneously turn into twenty billion copies. The infected parts were aboard BSL for several days, yet there were only ten or so SA-X. The SA-X fighting Dark Samus wouldn’t be able to just impromptu reproduce a few dozen times or something like that, obviously.
Excellent point. As for the scan, I must have forgotten that. D’oh.
Yes, it would take the SA-X several days to reproduce, but what I’m saying is that in a free battle, one SA-X might engage DS while the other’s grow. Still, it would take a long time, and DS would probably have blown away the SA-X by then. But then again… what if the SA-X had been reproducing before DS arrived, eh?
Thanks for pointing this out, I’m always welcome to commentary.
Dark Samus will beat SA-X.