Realize how little background information you supplied when asking this question.
What would I do? Keep the redline. Maybe buy a new stem/drops if you haven't (if you are still mustachio'd, unless you like them).

But, I find FG interesting, I value the newer bike w/higher perceived value, my area's hills are not difficult to overcome. I'm going to assume the Nishiki is a dime/dozen variety.

However, maybe the Nishiki is a high-quality model or has some incredible value to you, maybe you are moving to the mountains, maybe you don't like Fixed as much as you thought you would.

This is one of the type of questions that is probably best NEVER asked (twice in fact), particularly on BF where I'm amazed witty responses haven't been posted already.

You already know the answer.

Let me know if you need any more help figuring it out!