This doesn't seem all that hard of a question to answer. There is no guarantee that a new Mini will be coming any time soon (whether a few months from now, some time this year, some later year, or ever); moreover, a new Mini will cost you much more than just $150 - $200. If you need better long term storage today, I don't think the possibility of a future Mini should stop you from buying an SSD.
Also, a decent external SSD that works with a 2014 Mini will certainly still work with a future Mini, so it's not like you'll lose your investment if you upgrade.