I have hosted a lot of murder mysteries. You'll want to think about how involved you as the host want to be, the personalities of your guests, and how you envision the evening to go. That will all influence which one you get.
I find the PDF versions best. You can send the character books to your guests ahead of time so that they can dress appropriately and learn their character. Then print off everthing for guests who forget their copies and create gift bags with all of the 'supplies' they may need (depending on the theme, I've even replaced some paper items with real $1 store items. Squirt guns are a lot more fun in a wild west game than a picture of a gun, haha).
My personal favorite is: Murder at the Four Deuces Mature Download Murder Mystery Party Game
There is a mature and a tame version. But I like the mature version. They are unscripted, and include room for creative expression. "Every mystery is filled with plenty of opportunities for blackmail, bribery and other nefarious schemes for all your guests."
One of our guests' favorites was: Way out West – a murder mystery party game with a Western setting for 10 to 24 guests
Again, there is a tame and an adult version as well as room for creative expression (fitting for the wild west).