I'm trying to find out which open-system is better for programming, so those who know a lot of programming can you help me find out which is better pls?

Both are completely different and have different uses.

But personally, which is better for a begginer?

Arduino is better for a beginner, it's simpler.

>>4 more simple* Sorry to be a grammar Nazi, but that sort of irritated me for some reason. Possibly OCD? .-.

>>5 I don't think that's incorrect.

Would you say 'more happy' instead of 'happier', using the same logic?

Using more + adjective is not really automatically grammatically preferred. In this instance, it is quite ironic to use the phrase 'more simple', given the meaning of both words. I think using more sounds awkward enough to just say 'simpler', because 'er' can just suggest a higher degree, rather than higher amount. Some people think if it is at the end of a sentence, you should use 'more', but for a word like simple that ends in an 'l' sound, I don't agree. More simple is correct, but simpler is not wrong.
At the very least, there is no standard rule for this as far as I am aware.