• f/16 will give you sharper image than f/1.4. Yes, diffraction does kick-in at f/16, but it's still not as bad as the optical flaws that are pronounced at f/1.4 in pretty much every f/1.4 lens out there. (see: tests of your particular lens, resolution charts)
• Also lens coma and astigmatism are worse when lens is wide open than when it's stopped-down. That's important when you have point light sources (stars, distant street lights), otherwise: resolution will be your major concern (point above)
• f/16 will give you higher depth of field (by this: make focusing easy, which is especially important with autofocus lenses as they have can overshoot infinity (manual lenses (eg. Zeiss ZF/ZE) tend to be calibrated to end the focusing range at infinity making whole process easier).
• Longer exposure times might require remote triggering (or delayed shutter release), a Mirror Lock-Up (MLU) and a tripod to avoid capturing camera shake caused by a mirror and your hand holding the camera.
• When shooting during the night - you might get star trials with a (very) long exposure time (see: rule of 600)
• Long exposure images will suffer from added thermal noise. Usually it's not a problem, but the closer you approach 30 seconds mark the worse it becomes. It's especially important in some low-end cameras or cameras with known flaws, where you might get a brighter/more noisy spots due to electronics radiating heat. (see: dark frame subtraction) Best way to test if your camera got a problem with thermal heat from electronics is to put the lens cap on and make a 30 sec exposure at ISO6400 or so - if your image is uniform than it's fine, but if you see that for example right corner of the image is brighter than anywhere else - you will have a problem with long exposure photos.

In general - unless you intentionally want to achieve some desired effect - avoid extremes. Extremely high ISO, extremely wide open lens (or closed down), extremely long exposure - everything negatively affects image quality. Lenses very much like middle ground (again: refer to the tests for more information).