I think these are inside totally different cells. The Powercore II 20000 square shape tells me it is Lithium Ion Poly. Poly has a higher Voltage so more Wh per volume, it can be made in any shape but generally slabs so produces a square side.
The Powercore 20000 IQ2 has rounded sides, tells me it is inside 6 18650 3350mAh cells so the difference is in the electronics which should produce a more efficient and therefore faster recharge and discharge.
I’m more interested in the newer 20000 if it has faster recharge.
My actual needs are on long business flying trip days where I prefer to pack with under-seat bag only so I’m very volume sensitive, every cable and charger is picked selectively, a 10000 is not quite enough so I use a 10000 and either the Slim 5000 or the Mini+, so I’m lugging 2 ports 13350-15000mAh currently, and anything which is smaller is of interest. My 10000 is reducing its charge as its >18 months and used heavily already so I also need to buy something “soon” (next month or two) anyway.
I own 26800 - it is huge, its only really got used by family on long weekend off-grid type situations, it is overkill for me, I keep it charged for power outages, it rarely leaves home.