Hi nic,
There are always a lot of ifs when valuing any stamp.
IF a stamp on cover (cover = envelope) (envelope 'covers' or protects the communication being mailed / posted) is canceled nicely, that is it looks nice to the eye, which is rather subjective as laswabbie says, AND the hand writing is nice AND the other cancels are of interest AND the stamp is used in period AND the stamps used show a usage for a desirable rate (such as normal mail uprated to registered or international) AND the cover is 'clean' (free from stains or other blemishes) AND the back of the cover has interesting cancels (for me) OR the cover is addressed to or from someone of interest or has been used in a period of interest OR just looks nice AND you don't have any other covers from that period so this one is the best until a 'better' one comes along THEN it is better. IF you like covers that is. PLUS numerous other factors harder to explain.
Postal History can be fascinating.
To answer your first post: it depends on your collecting interest and your personal choice, what you prefer over another stamp/
A mint (unused with original gum) stamp presents well (looks nice when you first see it, your first impression, or even your second or third impression after close scrutiny and study) BUT some people do not care for them for different reasons. Usually it is valued more.
A used stamp (without gum, but sometimes a stamp, because of not being stuck completely down when first attached to the cover, still has some gum remaining, which is alright because, being used (with cancel) the stamp can usually be safely soaked and the gum removed) IS usually valued less than a mint stamp which presents better.
Whether mint or used OR unused or used OR canceled lightly or socked on the nose (bulls-eye cancel) AND whether you have decided to just collect unused or used THEN it always ends up with the question of how a stamp (or cover) 'presents' to the collector. You.
If you are concerned with value in years to come, especially to other collectors, then you must try not to get too personal with the stamp or cover and look at it with an impartial eye.
If I remember correctly you like cancels. Thus I would assume you would like postal history (covers, etc) that show usage within periods of postal history OR to and from interesting places or via interesting modes of transport (railways). That makes it an easy choice then. Look for that. Interesting cancels or covers.
My grandmother kept stamps 'on cover' (and other stamps in cover!) and had boxes and boxes and boxes of them. My aunt told me she kept them that way because stamps were 'worth more' if left on the cover. Well, they are IF some of the conditions above are met BUT if it is just one of literally millions of the same stamp used millions of times on cover (also depending on age and how many survived to this day) then 'usually' it is not 'worth more'.
BUT and this is an IMPORTANT but, if you do not know why a cover might or might not be important to someone (and here it is always hard to know everything) then it is best to keep stamps on cover until you know more and can then sort through them and at that time (possibly) keep only the ones that are 'valuable'.
Sorry for writing so much, but I think you can get an idea of how confusing and hard it is to judge these matters. There are a lot of concerns to be considered.
But who is to tell what exactly, 100 years from now, will be appealing to a stamp collector especially items of recent history? Things can change, opinion can change.
If it presents well then it is worth more. (Usually!)


