If Sovereign won, there wouldn't be a dreadnought. Hundreds (thousands?) of Reaper ships would be entering from Dark Space and would genocide the lot of us, so it doesn't matter. Thinking about it the other way doesn't really work, because Sovereign winning means the end of our existences.
The problem with talking about the tactical soundess of saving the Ascension is that we know nothing about the battle. What we see is that the Ascension is getting anihilated; their kinetic barries are down, the ship is heavily damaged, and it's sorrounded by geth.
In general, the issue with the tactical soundness of not saving vs. saving the Ascension is that we don't know what the battlefield was like. For example, where was the Citadel fleet, where was the geth fleet relative to the Citadel fleet and the Citadel itself, and where was the human fleet?
Consider these scenarios:
A) The humans come in from Arcturus. There is a single Mass Relay in the Serpent Nebula next to the Citadel; they enter exactly from where the geth entered. In front of them is the geth fleet, who are engaged with the Citadel fleet. Behind this engagement is the Citadel. For the Human fleet to reach the Citdal, they have to break through the geth, or swing around them. To entirely avoid the geth would take far too much time; they have to break through them to reach the Citadel.

The humans come in from Arcturus. There are mutiple Mass Relays (plausible, since the Codex says the relays come in pairs). The relay the humans enter from is different than the geth. There are two points of interest: one off to one side, where the Citadel fleet engages the geth. The other, the Citadel. The humans can
avoid one engagement and hit the Citadel directly. The Citadel fleet will eventually be anihilated, but not without heavy losses to the geth. If humanity defeats Sovereign, they could plausibly mop up the geth,
In Scenario A, it makes sense to hit the geth hard and save the Ascension, To reach the Citadel ASAP you have to break through the geth; engaging the geth to defeat them exclusively is stupid, but if you have to break through them regardless, it's logical to save the Citadel flagship while you're at it. Even if you try to move past the geth without engaging them, there's no guarantee this will happen. It's tactically sound to save the Ascension.
In Scenario B, the opposite makes sense. You could preserve your entire force
and reach the Citadel faster if you avoid the Citadel/geth battle. Yes, you're sacrificing lives, but that's war. The only victory condition for the Alliance is stopping Sovereign. Tactically, it makes the most sense to go directly for the Citadel when the arms open and stop Sovereign, then save the remnants of the Citadel fleet and/or mop up remaining geth.
Note that there is one scenario, IMO, that never makes sense, and that's the idea that the Citadel fleet could disengage the geth to save Sovereign after the Alliance saves the Ascension. The geth have no reason to allow them to escape. All that would happen is that instead of sorrounding Sovereign, you'd have given it extra firepower because not only is it destroying a ship every time it gets a shot, but the geth may well be ganging up on ships as well.
So basically, I think in actual tactical terms, the geography of the battlefield is all that matters. Of course, we have zero information on this (can't rely on the cutscene, IMO, because it actually shows things that directly contradict the condex entries on how ship battles & ship-to-ship weapons work). Since the soundness of tactics is totally speculative, I don't think we can really argue out a "better" choice.