just to add,
these days, it's easy to find adapter
for backward compaitbility (if your legacy device still uses mini-SD for e.g.)
but buying a mini-SD will certainly not allow to use it in newer devices since you can't shrink the card physically, can you.
Of course, gods know if there's going (probably just a matter of time) to be a nano SD card to replace the micro. I mean, 3 years ago, who have thought that SD card is a legacy, or mini-SD is not a futuristic size?