I would absolutely dump Sense 4+ for vanilla android.
Sense 4 is extremely slow and it drives me nuts because I have to work very quickly in my profession. The SMS application has to be one of the slowest apps on the phone. I hate having to wait for names to materialize in place of phone numbers every time I open the SMS app. The native calendar widget had to be dumped because it is slow to update and lagged. I had to replace it with the Google stock calendar widget to get smooth, quick performance with the app.
The pull-down menu is also too bare bones in Sense 4+.
Vanilla android is a heck of a lot faster and receiving all of Google's new features when they are released is a huge bonus.
I played with Sense 5 on a colleagues' HTC One and I was impressed at how much faster it is. HTC stripped out a lot of the "fluffy" things that make Sense 4+ so insanely slow. I would not mind trying Sense 5, but if vanilla android were an option, that is exactly what I would opt for.
Before anyone flames me on this, there is nothing wrong with my HTC phone. This is my second DNA, both of which were brand new when I got them. I dropped my first one and destroyed it. Best Buy provided me with a brand new one, not a refurbished model. Both of my DNA's act exactly the same, so I know the slow performance is Sense 4+. However, I am 99% certain what I consider slow or sluggish is not the same as what many of you consider slow, so please understand this is my personal opinion based on my needs in my profession.
This is my long winded way of explaining why I think a vanilla android option would be nice to have. Especially since HTC left an nearly vanilla android version on the HTC First after you disabled that facebook stuff. I know it can be done and I would love for HTC to provide the option. There is nothing wrong with having options.