the key difference is that the stock market is a centralized exchange and spot forex isn’t
the reason that’s so important, from the trader’s perspective, is that one (stocks) is a real, transparent, honest, objective market in which all brokers have the same prices and your broker IS actually a broker and wants you to win, not to lose, and there are no spreads at all, and lower commissions too, and volume figures are available (the only “voume” a spot forex “broker” can give you is of course their own volume, which doesn’t mean anything worth talking about)
you can get all those advantages with forex, too, but only by trading forex futures (which are centralized), not spot forex
harder to answer
if you want to trade stocks, then trade stocks; if you want to trade forex, then it’s a no-brainer that forex futures are better than spot forex, for all the hugely significant reasons mentioned above
nobody (in their right mind) who has enough trading capital to trade futures (realistically, you need at least a few thousand, rather than a few hundred, to do so) is going to choose to trade spot forex instead: that would just make no sense at all: one is all advantages and the other all disadvantages - you can’t get much more clear-cut than that