DC, if you're trying to see which types of banknotes appreciate best over time, the simplest way may be to compare Charlton's or other "trends" prices from previous years to the current year and then figuring out the rates of return. You could probably get back-issues of Charlton's catalogue or other pricing guides and then do the math yourself with an Excel spreadsheet. Myself, I have only Charlton's 4th (printed 1991) and 17th (printed 2004) editions, and it's remarkable how prices have [generally] increased between these two editions!
I have, so far, restricted myself to collecting Bank of Canada notes, I guess mostly because I grew up with BofC notes, so I have pretty much stuck to notes from 1937 and onward [the 1935's are currently out of my price range, at least in UNC!]. I am 30-something years old and grew up with the Multicolour series, so I am fondest of those notes, mostly for nostalgic reasons, even if they may not all be aesthetic masterpieces (however don't say a 1975 $50 note is unattractive; you may have a fight on your hands!)
I don't think I'll live long enough for the Journey series notes to appreciate substantially, so the most recent series in my collection are the Birds notes, but only replacement notes. Now that there are no longer such things as "replacement notes", but only "insert" notes, I see no reason to collect Journey notes. I don't give a flying DUCK about "insert" notes! There is no easy way to identify them except by poring through extensive lists of "known ranges" for inserts published in Charlton's or other publications. So for me, since there are no longer asterisks or X's in the serial numbers to
easily tip you off, replacement notes are dead
Anyway, strictly speaking about BofC notes, I am hesitant to consider them as "investments", as this word would tend to imply assets where the principal is protected and some minimum rate of return should be expected. I suppose holding a banknote could best be compared to the speculation involved in holding common stock - your principal is not guaranteed and you could lose money in the short run or even the long run.
For example, a 1937 $100 Osborne-Towers note is worth $3,000 in UNC condition. Sounds good, but this is really an average return of 5.13% for each of the 68 years that someone might have held this note!
As another example, a 1954 Modified portrait $50 note with the Lawson-Bouey signatures is worth $275 in UNC. The subsequent 50's came out in March 1975, so let's say the last of the 1954 50's were printed 29 years ago. That results in a 6.05% average annual rate of return.
It would seem to me that as "investments", BofC notes - both regular and replacement notes - are worth collecting only in UNC condition, and only in the lower denominations. It appears that in most cases, it is better to have $1 and $2 bills than the larger denominations: 20 $1 bills seem to appreciate better than one $20 bill for any given series.
As for Specimen notes, I just bought a Multicolour specimen set in June. Charlton's value for 2005 was $1,950 and for 2006 I'm told it is $2,150. I paid about $1,700 for my set. Another lesson - try to buy your notes before the new Charlton's catalogues come out each July and the prices go up...
However I collect Canadian banknotes mostly as a HOBBY, since to look at them strictly as "investments" may prove disappointing. I do own $50 and $100 bills even though it might have made more sense, financially, to put this money into a good mutual fund or stock.