Less explodey-end-of-the-world doesnt have to mean less epic. If Arthas isn't stopped it will mean the end of Azeroth, just in a more drawn-out nightmarish way than a Titan pressing a big red button. I often prefer the more personal touch than the unstoppable nightmare approach.

For example, the 3rd season of Buffy was always my favourite, although it was one of the few series that didnt involve saving the entire world in a dramatic fashion. Instead it involved stopping a (very well developed) villain becoming a daemon, and slayer-turned-evil. That personal form of evil, which Artha/Lich King seriously commands, feels more epic to me.

"Arthas kind of feels like a sideshow. I mean... Old God, destruction of the world vs some undead guy. Sorry, but Arthas isn't the biggest of those 2 threats. As far as we know now..."

If Algolon isn't beaten, Azeroth would be almost literally nuked, reduced to a smoking cinder by the Titans. (Or so I surmise, based only on what I read; I haven't watched any videos of the fight, let alone encountered him in a raid myself.) If Yogg-Saron is freed, faceless ones and their ilk would methodically slaughter all intelligent life while the god himself drives them to suicidal insanity, and then elementals would have free rein on the planet, which would make the planet inhospitable to life anyway. So the process would be different from that caused by Algolon, but the result would be the same from the perspective of anyone now living on Azeroth, just a little bit slower.

If the Lich King wins, like Yogg'Saron, his armies would sweep over the world, also methodically slaughtering all life in his way. However, while both of their arsenals include tactics that insidiously destroy their enemies from within - in Yogg's case, insanity; in Arthas' case, disease - only one of them also converts their victims to their side. (As far as I know, but even if not, at least Yogg'Saron's victims wouldn't become a threat to the living.) A victim of Arthas usually gets reanimated and remains conscious of his past life, even while completely in thrall to Arthas. That would be torturous, both for the victim and for anyone who knew them while living and now has to face them as undead. And that might never end. If Arthas succeeded at wiping out every last bit of (multicellular) life on Azeroth, he might drop his control of the zombies and call it a job well done (until a billion years or so go by and multicellular life evolves again), but then again he might set his sights on other worlds out there or on the Legion or just keep the Scourge around to lord over and sate his own ego.

And as for their respective chances of success, it's hard to gauge, and Arthas might well come out ahead. The Lich King started out with less power than Yogg, but the Lich King's power has grown, maybe even by stealing from Yogg himself, while Yogg has remained unchanged at best and is starting out from inside a prison that's still only partially compromised.

tl'dr version: I have no problem with saying that the Lich King is the ultimate villain of WotLK, and is more scary than Yogg'Saron.