Here's the point - if they're offering to buy you, they may well be doing you a favour. They could almost certainly do this on their own, and out-compete you.
You can tell by this move that they're interested in what you're doing, and if they have the kind of money you're implying to set up a team working on this, that money is there whether you guys come on board or not. So, much as you like being independent, can your little startup compete against a well funded competitor with a big name behind it?
Have a long hard think about that question, and try not to get emotionally attached to your company as you answer it.
As much as you like being independent, it may be better to take this opportunity, and move onto other things if you find it doesn't work out.
What I would suggest is to make a list of the things you really enjoy about being independent, and speak to this company about them. See if they are amenable to you guys having autonomy and control over this project. Find out what their long term goals are for it, and whether they align with your own. Get them to put in writing their commitment to the project and their goals, and to having you guys in charge - that way you should at least have a few years whereby you know where you stand, and can be confident of running the project the way you want, in the direction you want.
If that can work out, see this as a huge opportunity. Firstly you get the money and a bigger team to drive your project forward faster than you could have done before. You'll also get to make some good contacts, and probably learn a lot about how to run things like this, oh, and you get the cash too (depending on how big you think this will be, it might be worth insisting that your deal is partly profit related)
Then, if it does go titsup in a few years, and you don't like the way things are going, you're all in a good position to leave the company and startup on your own with a new project.