I wouldn't know about magic cards, sorry.

Stamps would be my choice because they have many advantages over other types of collectibles. Magic cards may share many of the benefits stamps have over other collectibles considering the similar format, but stamps surely have special perks magic cards don't.

Stamps have the advantage, that they are the number 1 collectible in Germany and many other parts of Europe and they have been forever. Coins and other things don't come close in terms of how widely they are collected and the bigger the demand, the easier it is to monetize if you have a worthwhile collection that is interesting to the collector base. There are stamps which one can assume will be interesting for many decades to come - there is for example a deep fascination with the Third Reich so stamps that came from Germany and the occupied territories during that time are generally sought after. What's also interesting is "catastrophy mail", that is mail that was delivered on planes or zeppelins that were destroyed while the letters themselves could be salvaged from the wrecks.

There are many nieche topics that stamp collectors could choose to base their collection(s) on, which may indeed suffer from vaning interest over time in the collector base, but there are also some other topics that will probably remain interesting in the very long run and thus demand will always be there and fluctuate less than the demand for other topics.

I know that magic is huge but will it remain so for another fifty years? (Assuming the absence of apocalyptic scenarios). I'm quite sure stamps will be there because they are carrying historical information and are an integral part of the first "reliable" long distance communication technology humans managed to make work (apart from books perhaps, though they usually had no specific individual as recipient in mind and thus really are quite a different communication technology).