I've got both the 1520 and the 620. I think they are all essentially the same knot with different handles and aesthetic band.
I used my 1520 yesterday with Proraso soap and had more than enough lather for 4 passes will some left over.
A deceiving little brush!
I like the handle on the 620, but it's a bit too short for me, so I'm using the 1520 more (same handle as 1470 just painted)
They really do get better with each use, mine is still breaking in after a week! The tips are getting softer and it's started to hold more lather.
I wouldn't go any bigger for a soap/face lathering brush. The 1305 I found just a tad too long (loft is 55mm vs. 50mm on these). If they made a 1305 with a 50mm loft I think that would be my perfect boar.
John