The purpose of a patent, whether provisional or non-provisional, is to give you the right to exclude others from making, using, offering for sale, or selling your invention. This is a pretty important thing; if you want to make sure all your bases are covered, you need to hire a professional who knows the ropes. If you write your provisional patent application, even though you can use a fill-in form, and accidently leave out something important, you`ve wasted time and money.
You asked about the difference between a design and a utility patent. Simply put: A Design Patent is for the Design - or the way it looks, and is based only on the design, or look of the invention. A Utility Patent is for the utility or "usefulness" of the item. It covers what the invention does.
Here`s what the USPTO says about these two types of patents:
1) Utility patents may be granted to anyone who invents or discovers any new and useful process, machine, article of manufacture, or composition of matter, or any new and useful improvement thereof;
2) Design patents may be granted to anyone who invents a new, original, and ornamental design for an article of manufacture
The clock starts ticking as soon as you discuss your invention to the first potential investor, and if a patent is not filed within one year of that date, you lose the right to get one.
US Patent Reform, if it goes through, will change to "First Filed" instead of "First Invented," which is the standard we have now. This will bring us in line with the same standard as the rest of the world. In that case, the "inventor log" wouldn`t mean anything. I`d keep the log for now, because the reform has not gone through, but also work to getting the provisional filed as soon as your research shows this to be a potentially viable product.
(BTW: IANAL, but have been reading about this quite a bit lately.)