The slower film traditonally has beter grain; the only thing is you are going to be trading a lot of speed, expecially at night. Remeber that for every "step" of film you go down, you'll need to slightly more than double your exposure time. At night, that can be very significant, in terms of the time that it takes to make your image. A couple of things that I've learned the hard way about night photography:
* Your exposure meter is wrong. I don't care if you have an F5 with RGB Matrix meetering or a N60, your meter will not work right. It will try to make something in the picture 18% gray- be it a light, the moon, or the night sky. The trick is to just try different exposures until you get a feel for what you need for the image that you want. Remember to bracket... one good image of the moon may take 10 shots to get right. (and speaking of imaging the moon- remember that a landscape or architecural photo may take a minute, five minutes or an hour to expose- but a moon shot will probably be in the area of under 15 seconds... take what your exposure meeter says, halve it, and bracket like crazy- two stops over to two stops under)
*Use a good tripod. Your exposure may be 5-10 minutes or longer; a little shake probably won't hurt if there are no lights in the picture. If there is a light in the picture, each shake you will get a trace on the film. If you are shooting a closeup of the moon (and you'll need the equivalent of a 600MM lens for this), each tinny little hint of a shake will be enough to ruin a picture.
*Use the time-delay if your exposure is under a minute. This way, you can have your hand away from the camera before the shutter goes up. If it's over a minute, be sure to gently, carefully, gingerly press the button if you have to. If you have a camera that will accept any kind of remote, use it instead.
*Don't even try to hand-hold at night . Even with 1600 speed film.
*The earth rotates. I know this sounds silly, and obvious, but it causes any stars in your picture to leave trails if your exposure is more than ten minutes or so. This can be a good thing, but you will need to remember it.
*Flare can be a problem- even at night. It's even worse, though, because you probably can't see it in your viewfinder. (it may be so faint that it takes five minutes to burn it into the film).
*Remember to under-expose your film if you want the lights to "pop" out; this will darken the suroundings.
*Remeber that film gets "tired" as it goes along... not that you're going to be calculating exact exposure anyway, but that the last 15 minutes of an expose captures less light than the first 15. And don't sweat it if you are off on an exposure by less than about 5%... if you are aiming for an hour exposure, an hour and thirty seconds won't really be any different.
Remeber that I'm just starting on this, too. The bad part is, expecially during the winter, this is habit forming. Your photography will improve as you go, but your sleep will suffer .