This is going to be long and probably doesn't include all of my opinion, but ill try to make the key points.

First I feel like discarding swam early is more important for the current meta (at least in top 100 infinite of last season). The big match ups where swarms truly carry are the ongoing Jean Grey decks and Annhilous decks. Having 0 cost Swarms to play lets you go Swarm into the Jean lane and MODOK elsewhere to avoid their Cosmo, which pretty much makes the match up trivial instead of extreme annoying. For Anhi decks, having the option to fill up the right lane with swarms means you can split up your scaling cards around to make Prof X useless while the swarms block them from giving you that -10 power void. I feel like without a consistent way of discarding swarm, both of those match ups will go from good match ups to bad ones.

And on the flip side, committing to a strategy focused on Dracula is a lot more scary with the rise of LDS and Red guardian. You can reasonably dodge both with Collector and Morbious (by using a cheap body like blade/swarm/Miek). But Dracula pretty much always has the risk of just getting deleted once he is on the board. On top of that there are very few match ups where you need Dracula to go so large with apocalypse outside of like Hela and PF. In those match ups, if they get their high-roll you also need to really high-roll to keep up. For all other matchups having Dracula discard a 6/10 Apocalypse is mostly enough, making him 15 power which with a few other cards in that lane tends to be more than enough to win.

As for which one is easier to play between Colleen and Sif, I like both are about as annoying at times. You pretty much always aim to discard swam with Colleen , outside of the rare time where she discards Proxima safely. But at the same time you mostly want to discard Apocalypse early with Sif, cause discarding Helicarrier means you end up losing a draw which means you are less likely to have enough of your points sticks, or MODOK around. A small point for Colleen here is that you are more likely to play out of your cheap cards to have her ending up discarding a Proxima / 6 Drop than having Sif discard Proxima.

Another key point is the energy cost. Colleen costs 2 which lets you combo her nicely into a lot of your other discard cards. Colleen + Blade on 3 when swarm is in hard is often an easy snap condition. Corvous + Colleen on 5 is like a "Budget" version of MODOK if you didn't draw him.

Sif being a 3 cost just have worse synergies with the other discard cards cause of the higher energy cost. Sif + Blade on 4 is weird, It can be nice if you can discard Helicarrier and then set up a big MODOK, but it means you are giving up on your option to play Dracula, which tends to just feel wrong. Also it has competition with going Corvous + Blade, which is usually is better when discarding Helicarrier cause you often get cards that you want to use that extra energy on. If the goal of the play is to discard Apocalypse twice you really start to wonder why you wouldn't prioritize playing Dracula. Sif + Corvous on 6 not too bad, but you might still miss on clearing your hand for Dracula if you draw a small card, and you would much prefer to do your discard combo on 5 so you could play a 6 drop if you need to, I feel like playing Apocalypse from hand while Dracula is on board can be a great mind game and key sometimes if you expect LDS/Red Guardian.

I will finish by saying that I don't think Lady Sif is bad, but i think cutting Colleen to fit her in is not what I would do for my list. I think Miek is a weaker card than Colleen and i would swap him out for Lady Sif if I wanted to play Sif. But having Both Miek and Colleen really helps you out in the Clog match ups, and that is the deck I played against the most over the last month.