In Karakura-cho there are currently 4 Captains engaged in battle against 3 Arrancar and 1 Fraccion with 2 Captains in reserve and 1 Lieutenant currently combat capable.
Assuming worst case scenario of draw with all the currently fighting captains removed from combat, the nature of shounen series like this means that they're probably going to do enough damage before going down for Yamamoto and Komamura to mop up fairly quickly. That leads to a 2-on3 fight in Aizen's advantage.
Soul Society has significant reserves not currently arrayed on the field of battle. The group in Hueco Mundo currently has to fight Yammy but considering there are 4 Captains, including two who max out their offensive power rating and all their Lieutenants including, Renji who along with Ichigo are close enough to Captains for our purposes, Ishida who once beat a Captain, Rukia who probably would have been a Lieutenant if Byakuya hadn't been sabotaging her promotion chances and Chad who has to be a Lieutenant-level or higher given the way he beat down Ganbantein. Yammy is deader than disco and with Orihime on the scene any resulting injuries are going to be taken care of easily.
It's true that they are technically trapped there, but even assuming Mayuri can't MacGuyver a way out of there (not beyond the realm of possibility) Ichigo has made allies. Pesche and Dondonchaka are fairly firmly on Ichigo's side on Nell's behalf and at least one of the Privaron Espada is still alive. Even if they won't help fight they could still open a Garganta to where everyone else is. This means that, even just counting Captains or equivalents that's 9 or 10 to 3 in Soul Society's favour (counting Ishida as a definite and Chad as the swing number).
But wait, there's more! That's not the end of those who would oppose Aizen. Though they are technically exiled criminals (or at least three of them are) they have shown a willingess to co-operate with Soul Society in recent events, probably because Aizen was involved in their exiles. I am of course talking about Urahara, Yoruichi, Tessai and Isshin. That's another 4 former captains putting the count at 13 or 14 to 3. Possibly as high as 15-to-3 if Ishida Ryuken can be persuaded to enter the fray.
In essence while the forces <i>currently</i> fighting Aizen are in a worst case scenario somewhat over-matched once Soul Society starts bringing in their reserves (and the impediments to doing so are minimal as things stand) the balance swings way over the other side of the scale. And since IMNSHO the evidence indicates that the Vizard are still more likely to come in on Soul Society's side than Aizen's side I can't see how he's getting out of this short of running away with his tail firmly between his legs.