People choose by asking which language is most useful, then quit in month four. Usefulness gets you started and almost never gets you through the middle, where the work stops being fun and there is still a long way to go.
Pick the one you will still want in a year
The strongest predictor of success is not aptitude or difficulty. It is whether you have a real reason to keep going: people you will talk to, a place you go, work you do, something you want to read or watch untranslated. A hard language with a genuine reason beats an easy language with a vague one, every time.
Difficulty is real but secondary
Some languages take two or three times as many hours for a speaker of yours, because of writing system, grammar or distance from what you know. That is a real cost and worth knowing. It matters less than motivation because the hours only get spent if you keep showing up. Use difficulty to set your expectations, not to pick.
Test it for two weeks before committing
Spend ten minutes a day for a fortnight on a shortlist of two. Learn to introduce yourself, count, and say a few things about your day. You will discover quite quickly which one you enjoy being inside. That feeling is more predictive than any argument about global speaker numbers.
The practical tiebreakers
Availability of material, whether you can find people to speak with, and whether you will hear it in daily life. A language you can practise without arranging anything is far easier to keep. If two options are otherwise equal, choose the one that is already around you.
Choose the language you will be glad to still be learning in twelve months. Every other criterion is settling an argument that motivation will decide anyway.