I'm of the mind that you never take the first offer. Always negotiate, especially if you know that you're worth more than they're offering. I say good for you to ask for more, considering the experience you bring. Stick with it, and if it doesn't work out, they'll be more jobs around the corner. You don't want to work for someone that's going to low ball you any way. If the relationship doesn't start out with mutual respect, it can only go down hill from there.