Yes, that's what I meant by 'cotton topcap' - I don't remember what the official name is. It's to help support the wick if you use cotton, otherwise the wick will tend to lay down and lose contact with the filler since cotton lacks the rigidity of silica or Ekowool.
I ran my Heron for a while with no filler but the filler is the crowning glory of the Heron and it's really not that remarkable if you don't use it. I was impressed by its ability to vape literally the very last drop in the tank though
The filler (sera wool) was the hardest part to learn and I still struggle with it from time to time. If I overfill it, it will leak out the air hole. I usually dismantle and clean the gpin when I clean the atty, othewise I don't take it apart. I've found if I do flood it, I can tip the air hole over a paper towel and gently blow on it. This will usually clean out any juice that would otherwise leak.
I have an Erlkonigin as well and it's fantastic. If I were forced to, I would have an exceptionally hard time choosing between the two. I think you will be very happy with either. They both have benefits and drawbacks, but they are 2 of the best attys you can buy. Since I struggled with the filler material for so long, I think the Erlk is easier to build on, but other users have had no issues with the filler and I feel a bit stupid because of it.
The vape on the Erlk has more raw flavor and is easily on par with the Kayfun. The Heron is smoother and personally I prefer it. The Erlk has some benefits, like being able to see the juice level (something that's lacking with a filler build Heron) but it's more tedious to rebuild as the Heron can simply be pulled apart. I do love the nano kit on the Erlk. It is nearly as small as a dripper yet holds over 3ml of juice and is unbelievably lightweight.
I would also like to mention that all the issues I had with the KFL build design were fixed with the Erlk. Namely the pos blocks' tendency to twist, the threaded tank (the Erlk has a threaded chimney instead) and the little screw to refill with (the Erlk fills from the top with no tools).