I fired a few rounds through a 901SE (modular) upper on my 901 lower. The difference in weight between the two uppers are very apparent when you handle them. The 901S handles much better than the Armalite and the LMT MWS I previously owned. The 901SE feels very close to my M4A1 SOCOM barreled carbine.
The 901S has a quad rail that is narrow and tall. This isn’t a bad thing, but it has a different feel than most handguards. The 901SE slick handguard feels much rounder in the hand, and I find it much more comfortable in the hand.
The new slick forearm on the 901SE makes the gun noticeably lighter. When I was firing from the bench I did find the rifle moved more than the heavier 901S. In offhand rapid fire the rifle did move more than the heavier full quad rail version. It is still very pleasant to shoot, and very controllable.
I think the standard quad rail 901 is a very handy controlable gun. The 901SE modular rail gun gives up very little by way of control ability and feels and handles so much more like a heavy barreled M4. If you are thinking about getting a 901 and plan to mainly shoot from the bench, get the 901S as it moves less when shot. If you plan to carry it and do run and gun, get the lighter 901SE.