I have both but only purchased my black Mini a couple of weeks ago and haven't used it yet. My medium black lambskin flap is several years old and far softer than my new Mini. I'm so glad I have this bag, it doesn't get much use, special occasions and going out mainly. It's still more or less as new. I much prefer lambskin to Caviar, the feel and lustre is just TDF and what Chanel is all about for me ❤️ Im sure the finish of lambskin has been changed in the last few years to make it more durable. This suits me as my Mini will get used regularly and when worn x body it does need to be a little more durable. It feels different, yet still lovely, soft and puffy with a bit more sheen to it.
I feel they are similiar bags but I use them for different purposes. I'd say if you main use will be popping it into another larger bag the Mini will be ideal especially if you use a hands free style mostly. It's comfortably worn x body, and you can either shorten the chain (with a hook inside) or double it for shoulder use. The chain on the Medium Flap is far shorter and can't be worn x- body.
I don't find the Medium as versatile and if I had to move one on in the future it would be the Medium as I can't wear it x body. The Mini also fits everything I need to carry light, small purse, phone, lippy, sunglasses and it's actually easier to get into than a double flap of the medium, really I guess the Medium is superfluous for me as I could use the Mini for everything I use the Medium for but not vice versa ........ but it's not a decision I'll make just yet
I've been told and heard so often the black Minis are difficult to get hold off so if this is the bag you would get most use out of right now I'd keep it. In the future if you feel you still want a larger one as well, the medium is from the Classic range and available every year, or you can always keep a look out for a "pre loved" and definitely keep the kidney!!
One other thing to consider is maybe buying the Jumbo rather than the medium in the future. There is a bigger contrast in size between the Mini and the Jumbo and it doesn't cost that much more than the medium. I find it's more versatile as it fits considerably more and can be worn x body if necessary. If I was only allowed two Chanel bags
it would be Mini and Jumbo size