Diffuser or absorber behind listening position....and is it worth doing??

No, it's the density of the material.  I think it is lbs per cubic foot of material.  So a 6lbs version of the 2" panel is going to be twice as heavy as the 3 lbs.  ATS Acoustics sells the 3 lbs version here:

https://www.atsacoustics.com/item--Owens-Corning-703-Case-of-6--1004.html

A box of 6 of the 703 2" thick 3 lbs version is $72.  Add about $50 for shipping gets you a total of $122.

Okay, hold the presses.  I was talking on assumptions.  The 703 has a density of 3 lbs.   It is the 705 that has a 6 lbs density.  So if you order the 703 from your website you should be good.

There is absolutely no difference in acoustics between the 3lb 703 and the 6lb 705.  The 705 is just going to be a heavier object to mount on the wall.  In most cases, you actually want the lighter 3lb density.

It isn't until you get to the Roxul Rockboard stiff/heavy panels that you will find any differences.  Their 2" panel will absorb more midbass frequencies (around 125 hz), but they are extremely heavy at 6lbs or 8lbs and they tend to sag.

I have a room almost exactly the same size (11 feet by 17.5 feet).  Diffusers can be beneficial.  However, be aware that QRD type diffusers (quadratic) need very careful implementation.  QRD diffusers can do weird things to the sound if they are placed very close to your listening position (like within 6 feet).  In this situation, the QRD diffusers can boost some midrange frequencies while cancelling out other midrange frequencies.  This creates a very bad and unbalanced sound. (this was from testing QRD diffusers on the side walls).

I was successful in placing two 2' x 2' QRD diffusers in the middle of the back wall.  That was the only place these QRD diffusers really worked.

I also have two 703FRK panels in the rear corners and another two 703FRK panels mounted at an angle on the front wall / ceiling edge.  These panels are actually "reflectors" for mids/highs due to the foil on the front surface. 

The rest of the panels are absorbers or tuned membrane bass traps.

The variety of "diffusers" that GIK applies onto their panels are not really true diffusers.  They are primarily "reflectors" to prevent much of the sound from being absorbed, but still keeping the panel useful for bass frequencie absorption.

The GIK "gridfuser" is sort of a QRD diffuser but without the channel slots.  Their PolyFuser is like a broadband reflector that reflects all angles (sort of works like a diffuser but more forgiving).

Be careful with how much absorption you add.  You can definitely over damp a room and suck the life out of the high frequencies (thereby removing excitement).

Chorus recommended ua-acoustics for a really cheap solution.  Be aware that these are just foam panels.  They are NOT the same thing as GIK panels which use fiber material similar to Owens 703 and the like.  I have tried foam panels before and they really don't do what you want them to do.  They don't absorb all frequencies and I actually had worse results with foam panels.

GIK Acoustics really is the cheapest "GOOD" solution here.  There is a company called ATS Acoustics that has panels slightly cheaper than GIK, but they use a hard-back panel frame (essentially a 1/4" piece of plywood on the back).  This is not the design you want because I have found the hard-back will resonate with the material and create more echo/vibration.

The 3lb 703 is excellent choice to work with because it is super light and it also does not sag.  I wouldn't recommend the 6lb 703 because it's a lot heavier and doesn't really do much different.  For my room, I just took a 703 2x4 panel and wrapped with with Guilford of Maine black fabric.  I just used spray glue on the back side to hole the fabric closed.  I don't made frames for my panels, but I also don't hang them (I just sit them on the floor/wall or on top of a bass trap agains the wall).  You can make frames if you want.  It's really not difficult.  Just get some light weight pine 1x2 from Home Depot and cut them.  Then glue/nail/screw the frames together and inset the 703 into the middle.  Then wrap with fabric of your choice.