It seems that the vast majority of folding bikes with gears ship with rear derailers.
The advantages of derailers are (1) generally less expensive than internally geared hubs; (2) you can inexpensively replace a cassette and change your gearing, but if you don't like the gearing of your hub, you have to replace the whole hub.
The advantages of the internal hubs are (1) much luss vulnerable to abuse or damage, whereas it's not hard to bend the cable jockey arm on a derailer, (2) requires less maintenance; (3) you can change gears while stopped.
My road and mountain bikes have derailers, of course, but I'm worried about having a derailer on a folder. I'm concerned that the constant folding/unfolding/storing, etc., will result in tearing up the derailer. Am I paranoid? Who on this forum has damaged their derailer? Or knocked it out of aligment so that it required maintenance? There are sure lots more choices if one is wililng to buy a folder with derailer.