Make good and sure that whichever lights you buy are DOT approved. This takes in lamp wattage/intensity, and beam pattern. Each state is unique, so check with your local authorities.
High Intensity Discharge headlights are definitely the trend of the future, they create very little heat, draw very little power, and produce a very high lumenesence light. However, they are extremely cost prohibitive, and given the *ahem* frugal nature of most folks, just not a viable option right now. Yes, I know the cost is going down, but I would wait, there are still some issues to be resolved with them, and indeed the cheap ones will tell you in very fine print that they are not approved for highway use, so basically you have a worthless turd at that point. Yes, cops will nail you for it, they are doing it here.
I did an H4 conversion on my Bronco, and I like it really well. I used the Hella lamp assembly, which comes with a new reflector (DOT approved, and the bulk of the cost) and 55/60w bulbs. These are still an halogen lamp, but there are xenon and other 'rice' light colors available. None of the sealed beams I have run across have such a good beam pattern, and to me that is a big consideration.
I got my Hellas at 4Wheel Parts Wholesalers, but I am sure that one of your favorite hot rod suppliers might carry them also. I paid $35 each, and with tax and everything I had about $75 in the lights, another $25 or so from the parts store for a fuse and relays, some wire and you are at ~ $100 total. When you install them be SURE to upgrade your wiring, and add a relay to isolate the power from the factory harness, and it will improve your light output tremendously. Here is a good article explaining how to wire them up, I know, it's not a Mustang, but still a Blue Oval...
http://home.comcast.net/~galenwea/head_light_upgrade.htm
Take your time and do this right the first time. Remember that your car is your pride and joy, and that many car fires are caused by shotty wiring done by lazy people. Take pride in your work.