true, but swimming against the tide can either pay off really well or bite you really hard

One important thing I would also add here. Think about your daily work in both companies: how valuable it is, how does it impact the company in general.

Right now I work in a huge corporation. The work I do is not THAT important. I mean, it's important, but if I ever decide to leave, that will not affect the company a lot. They will find another engineer and that's it. Such model is good for business, I guess. But, it's important that every employee feel how valuable they are. Not just feel, they should exactly know, that the work they do does really-really matter to a company. This is just imho.

exactly! many people dont realize in big companies there are (relatively) very few people that matter. rest are just cogs in the machine. doesnt mean you can grow up from being a cog, but what you say is true.

B wants me badly. My skill set is very niche. B needs me to propel them forward and in a different scenario, thats a great situation to be in. But I am very hesitant of how motivated B is to transform to the next level.

It also depends on what you really want. Make a list of pros/cons and then go from there. Think hard where you see yourself in the future. Go with the company, which can help you to get there.

You can always quit and find another job. Not a big deal nowadays.