First fired my Dad's Uberti/Marietta Arms 1861 Navy back in 1969. Remington and Navy Arms (RWS/Dynamit Nobel) caps were about all that was available at the time. Remington #10's always worked fine & still do to this day. Seems that the standard Uberti & Pietta Nipples are all made for #10's.
Now, about those RWS caps - the part # even today is #1075, which makes me think they're supposed to be #10s. Comparing a 30 year old tin with some new RWS caps, they're both the same size - too big for the #10 nipples on the 43 year old Uberti Navy, another 35 yr old Uberti Army, and my just-made-this-year Piettas. They fit just like #11's on a set of the Treso/Ampco nipples I installed on an 1858 & function just fine. But they're definitely not #10 caps.
Today, my favorites are whichever is cheaper - usually the CCI #10's over at the local Bass Pro Shops.. Never had any misfires or problems with either CCI or Remington in shooting performance, although the Rem caps don't feed as smoothly in my cappers.
Take a look at the Remington cap & you notice that it's been stamped into a cup shape & the open end edges are ragged, while the CCI caps have a nice squared off open end. That jagged edge on the Remingtons makes them want to fall over instead of just sliding when advancing in the cappers.
All caps are ribbed, which allow them to expand a little when seating on a cap. If you need to pinch your caps to get them to stay on, you need to use smaller caps. In all my years of shooting Colts & more recently Remingtons, I've never had a cap fall off & into the inside of any of the guns & have never had any problems fully seating one with either finger or a small wooden dowel.
Even my 35 year old Stainless ROA likes # 10 caps. From where I sit, the only use I've found for #11 caps is with the Treso/Ampco nipples installed on a Remington Sheriff's Model.