Just started a job and now that other company is calling

I think I know the answer to this, but I feel like I need to hear it from other people to really sink in.

I was interviewing with 2 companies early Decemeber. Company A was seemily to offer more money with a more stable company with many growth prospects but they did not have approval to fill the role and couldn't even bring me in for an in person interview. Company B offered less money in a start up with a very exciting new role though I don't know where it will go from here or how I will recoup the 30% pay cut, but they offered me a job in a week.

So I took the bird in hand, just started this month.

Now Company A is calling me telling me they want to move forward - they had to change the role to accomodate the greater skills I offer, etc. all nice things. I sure would love to make more money, but I feel I've made a commitment here.