Bookshelf vs diffuser panel. Who wins?

full and healthy bookshelves perform a similar function to room treatment and (more importantly, imo) makes a space cozier and more comfortable to be in.

i have nothing against professionally treated rooms - they sound amazing - but i personally prefer a comfortable space surrounded by the things that i love and value to some sterile, professionally treated, Serious Listening lair with a single chair and 50k in electronics on the floor surrounding the obelisks from 2001: a space odyssey. i could hit the lottery tomorrow and would still gladly make that sacrifice (or alleged sacrifice) in sound quality 8 days out of the week 😂

Personally I can't stand bookshelves in rooms. You have to fill them with books or assorted pictures and objects so they will hopefully interest visitors. U are Stuck with them.

Much prefer the blank canvas of a wall that you can decorate much more creatively.

What wins?  Artificial ficus trees win.  Get a few 5' - 6' artificial ficus trees and use them as diffusers.  Much more effective than bookshelves or wasting money or time on diffuser panels.  I have over a dozen in my listening room and they are great.  And have a VERY HIGH Wife Acceptance Factor value.  

Books work well! if you tilt the 3 book forward, DA Scanlon will come down the elevator fireplace!

Books work well enough...so do a stack of records. So many functional ways to use traditional decor to accommodate acoustic benefits. For example I have no highly reflective surfaces in my living room except a 60" monitor on the side wall. I cover it with a wool throw blanket when listening. No picture frames with glass. All artwork is canvas or wood carvings. It's not perfect...but it looks like a living room and it works well enough to get some imaging and focus.