Hi end microphones capture more extended frequency ranges than their less expensive counterparts. Therefore they are able to capture more in the way of subtleties. If you are recording direct to a camera that limits and/or compresses the audio data (most of them do to compensate for having inferior preamps, etc.) many of those subtleties are in danger of being lost or, conversely/perversely, overly hyped.
The Sennheiser MKH-416 is a fabulous mic; in fact it is the next mic that I will purchase. However, if you want to use it on the set you should hire someone with the knowledge and experience to use it properly. If you go to the expense of renting or buying the 416 and hiring a person to use it properly you should then record to a medium that will take advantage of it (higher bit rates/sample resolutions).
Also keep in mind that hi end shotgun mics have better off-axis rejection; they record more of what they are directly pointed at and less of what is around the aiming point. So your boom op really has to be on his/her toes so they record the dialog and not whatever is two feet to the left and ten feet behind the talent. Improper use of shotgun mics results in that “roomy” sound that is so prevalent in indie productions. (I have always thought that shotgun mics were misnamed; they really should be called “sniper” mics as they have to be aimed very carefully to pick up the best sound from their “target”.)
Here’s two links that will give you lots of great information about location sound and film sound in general.
www.equipmentemporium.com
www.filmsound.org
My recommendation is to purchase the Sennheiser ME-66 kit that has the mic (ME-66), boom pole, shock mount, blimp, dead cat, cables, etc. The ME-66 is widely used; it sounds very good and it can take a beating. You can buy the kit for less than $1k, which is what it would cost you to rent for ten weeks. Record direct to camera and make the volume of one channel fairly “hot” and the other fairly reduced. This way one channel will pick up the “quiet” stuff, and when that channel peaks out on the loud sections you have clean audio from the reduced input level channel. This set-up is pretty good for very low budget indies and the “run & gun” situations of guerilla filmmaking.
Great sound is not just the equipment; just like the rest of filmmaking it's attention to details. Scout your locations carefully. That road nearby is nice and quiet on Wednesday, but it may be a major traffic headache on the weekends when you’re shooting, or it could just be bad from 3pm to 7pm. Turn off, or better yet, unplug, AC, refrigerators, TVs, stereos, computers, etc. Even when turned off they generate noise as they are really in “standby” mode. Put sound blankets on and around things that cannot be turned off. Sweep floors; that little piece of grit sounds like a friggin’ avalanche when it gets to audio post. Make sure that your crew wears broken-in cotton clothes and sneakers; once had a PA who wore corduroy pants and a silk shirt – she sounded like a hack saw and rustling leaves in the headphones. Try to put carpets on wooden floors and/or have your talent wear soft-soled shoes. You get the idea…
I’m sure that others will chime in with advice as well. I’m here if you need me.