Most people will tell you that the best method is to do a clean install, after backing up all your valuable files to another HDD, flash drive or whatever.
I, however, simply upgraded from Vista to Win7 on my desktop computer. It was quick and easy, all my settings and valuable files (although backed up) remained unchanged and I have had no problems whatsoever.
On my ageing laptop I did a clean install and have had no trouble with that method either.
So, really and truly, the choice is yours.
The only difference I have ever found is that my copy of Flight Sim X, with many addons, loaded considerably quicker in Vista than in Win7. Reading various threads in Flight Sim forums shows that this is a common problem with Win7. Would I go back to Vista again? No!!
Good luck in your change of OS.