They're both hyper carries, which is to say they're carries with weak early games.

Ashe is more oriented toward utility and team fighting.

Sivir is oriented more tower utility and survivability.

Ashe is more friendly to newer players.

Sivir is much less so, but it comes at a trade; a Sivir that can time her spell shield can absolutely wreck some bottom lane supports.

I disagree that either are "hyper carries". The old Sivir could be considered a Hyper carry, but Ashe? Never.

Hyper carry potential (to me) means that it's someone that can 1v5 the enemy team, get kills, and survive. Hyper-carries off the top of my head are: Trynd and Poppy. Both have the ability to auto survive 1v5 if they're good, as well as amazing damage output.
Nasus is also a hyper-carry if you get to late game with him. Similar to Veigar and Sion.

Currently, Sivir and Ashe are just regular carries. They deal good damage and the likes. Pre-Rework, Sivir's ricochet would basically amplify her auto attack damage by 5x which is absurd, and why she could be a hypercarry just on sheer damage potential.

there is no better one...they both have their pros and cons

ash (pros)
-amazing slow
-has stun
-instant ward
-can initiate a team fight
(cons)
-no real escape ability
-no that burst
-no steroid
-no gap closer

Sivir (pros)
-has a gap closer
-bursty
-block magic
-can run away fast
-has steroid

(cons)
-no cc
-can initiate a team fight really...
-no escape or gap closer with out ault

What do you guys call "steroid"?

I've played very little Sivir, but my main adc is Ashe. She's one of the easiest champs to crush with once you learn map awareness and patience. If you are starting out, she can be way squishy and you might die a lot if you're careless

I think she has great escapability if you use her abilities right.

"steroid" refers to any ability that would increase overall DPS.
This means AD boosts, AS boosts, and Armor Pen boosts. 1 Shot abilities don't apply. This refers to abilities that give long duration buffs.

Sivir's steroids are her Ulti which gives her a ton of attack speed. And her W's really low cooldown is effectively a steroid too.

Ashe has no steroid.

For other Champs:
Ezreal's Passive grants him AS.
Tristana has an ability that grants her massive AS
Graves has an AS boost in his dash.

Yi has an AD boost on his E
Xin has an AS boost.
Gangplank has an AD boost
Poppy has an AD bonus

But yeah. Those are steroids.

I like both to be honest. I wouldnt limit myself to one.. i usually pick sivir to try and counter the lane.. example, a blitz support to try and counter his grab with sivirs E... Thats not to say that it is an AUTO win if u pick sivir vs a blitz, you still need to know when to use it.

ashe i use when im vs a good range ad carry, or when i feel i will get played aggressively against; the slow really makes people think twice about overextending.

I think a vital point about ashe is knowing her limits. Ive seen people be TOO passive with her, and ive seen people be too aggressive. Know who you can kill alone, gauge the other teams aggressiveness, ashe is a great initiator. But dont chase just to chase, sometimes its better to not chase that bruiser who is almost dead if its gonna lead to your own death.

Sivir on the other hand is really about managing your ult and is more ability (Q,W,E) based.

Graves/Corki/Ezreal best atm throughout the whole game.

Sivir and Ashe are both very different kinds of ADCs. Sivir is a bit more AD-Caster-y like Graves, and Ashe has a lot of utility in her kit.

I, personally, like Ashe more because I like her playstyle. I don't particularly like playing Sivir (or Graves, for that matter).

Ezreal is borderline OP. Graves and Corki are popular, but they're ADCs that have their own kinda thing they do. The best right-click champ is probably Kog'Maw.