Sheet Replacement Note Or Special Series Number Note, Which To Collect?

Rare prefixes, solid #, 001 are the ones to adquire at the right prices, most collectors do not know what sheet replacements are ?

"What to collect?" is a pretty huge topic: in fact, I dedicate my entire Notaphilyc Culture hobby site to the many approaches to our hobby:
https://sites.google.com/view/notap...ng-banknotes

To address the other parts of your question:
Sheet vs Single Note Replacements (SNR):
This term came out after brick searchers discovered different prefixes showing up at intervals in the Birds & then Journey Series. They first called them insert replacements b/c they weren't designated with an X (like most replacements from the tail end of the Birds series). These sheet replacements were fairly popular b/c collectors realized that they were difficult to ID (unless you had a Charlton) so most would be overlooked (whereas X notes were easy & printed in large quantities before the inserts). BTW: A good sheet(s) (replacement insert) would have replaced a defective sheet (regular prefix).

SNR's were discovered much later on with the tail end of the original Journey's since so few were discovered. They quickly realized that the printers had the technology to literally replace a single note if it were defective. Although you'd assume all SNR would be rarer & in more demand than sheet replacements, this has not always been the case, as some sheet replacements were very scarce & rarely seen. Insert replacements were stopped around 2015 during the issue of the Commemorative $20 I believe. So we can find them in all the polymer series but the commemoratives & 2018 TENS. More on this topic, I cannot really elaborate on, as I don't actively collect inserts. I have some sheet replacements b/c of swaps/trades & my own personal finds. I also sometimes bought a replacement that was part of a signature change-over as this is more what I chase.

Some collectors don't seek inserts & rather collect X replacements:
http://goccf.com/t/355833

Also there are those who just seek asterisk replacements. I started a thread for those here:
http://goccf.com/t/355832

Special serial numbers:
Radars, repeaters, rotators, ladder serial numbers, low # (under 1000) are all very popular today. I created a page on my NC site for special serial numbered notes:
https://sites.google.com/view/notap...rial-numbers

Personally, I really dig special # notes b/c I have searched for them in bricks of 1000 & have struck out & found them to be so elusive to find.
Here's an old thread on some tough notes found in circulation (nice low # , radars ,etc):
http://goccf.com/t/318764

Well, I hope that answers your question. Happy hunting!