Hi guys! I need your help.

I'm currently facing a big decision.
I've got a job offer a couple of days ago. The new job would involve an 80% raise to my current salary, and I would make another step on the hierarchy ladder.
BUT
The new place is not a software development company. They have a small team working on internal stuff, but they are basically maintaining a 12 years old garbage.
My job would be, to design the new system from the ground up. At the moment, the new system has to do the same things as the old one, just faster and better. Then they'd like to extend it further.
The first part is not challenging, but the things that they planned in the future sound interesting.

The problem is, that my current company just got a new contract and I'm supposed to conduct the deploy (speaking with their managers, prepare their sites for installation, and install). And since it is a small startup, the deploy depends highly on me.
If I take the new job, then I have to start in February which ultimately means that I screw my current company real bad. They'll probably survive, but they might lose this contract and/or lose money.

If I do what makes economic sense, then I take the job. (fuck it's almost 2x as much!!) But I have mixed feelings about it.
I've got 48 hours to decide.

What do you guys think?