Why not do both? I'm going out a few days before to ride some of the Italian alps. Then etape.
Having ridden it last year, the vibe is really great because you are riding past people that have set up their camper vans for the tour. They are cheering you on, handing out water and cokes.
What's the appeal of doing the Etape over, say, a self guided holiday out in the mountains, out of interest?