Having studied both, it's pretty clear to me which is easier - Japanese! It's true that Japanese has three writing systems, but two of them are not difficult to learn and they actually make the language easier because it means Japanese is not 100% character-based like Chinese. The tonal system in Chinese makes its pronunciation much harder too than Japanese. Even without the tones the pronunciation of Chinese is more difficult, but the tones just complicates it even further. I agree that Japanese grammar is generally a bit more difficult, and you also have the kun/on problem in Japanese which you don't have in Chinese, but when all is considered I can't see how anyone can really say that Japanese is more difficult than Chinese. It just isn't!

It depends on what skill you want to learn. If it's reading only I recommend Chinese. Japanese writing involves three separate writing systems. That's a lot to learn. Speaking is another matter. Chinese has much less grammar to learn so that makes it easier than Japanese, but the tonal system in Chinese takes a while to learn. On the whole I would say neither is better for an arabic speaker. John