1) Buy the filter for the largest lens you own or think you will own. ; You can buy setup rings for dirt cheap, and they allow you to mount another size filter on your lens threads. ; You may own multiple step up rings if you have multiple thread sizes though.
2) Don't think about either of those filters for the 50 - do the size for your main lenses. ; If you are doing landscapes, you'll probably want very wide or very long, not "normal". ; Also, if you're using a tripod (heck in daylight it doesn't really matter as well), you're going to probably be shooting stopped down, not wide open, and most lenses stopped down at a certain point provide about the same image quality. ; At night - the same thing - unless you are handholding - in which case you wouldn't want anything that reduces the light coming into your camera, so you'd be shooting wide open with the 50 anyway.
ND filter for night? ; Unless you're using a graduated ND filter you probably would use the ND filter during the day because you can't stop down your lens enough AND lower the ISO enough to get the shutter speed you want. (like several seconds to blur water. ; BTDT.) ; Never used an ND a night as you probably will already use a tripod and you'll have several second long exposures already. ; Bright scenes - okay, yeah maybe, but remember an ND filter will also prevent the shadow areas from being exposed, so all you are going to do is take longer to get the same shot. ; If you want to prevent highlight blow outs and get shadow detail, that's what HDR or grad ND filters are for. ; Or by shooting RAW and pulling the data from the shadows.