The programming language, "spin", is rubbish compared to the C/C++ of Arduino. Yes, you can use assembly but it's a pain. Also, I believe using "spin" the individual hubs are actually slower than an Arduino.
Some people have got C++ working with the propellor and it can use all 8 cogs, no idea how well it works though...
Yes, there are two C compilers one is free (made by some users) and one is cost $80(Made by Parallax).
both are excellent.
3) The programming library is not as helpful. Maybe it is there but it is not as well organised.
It is smaller but it has every thing you need I.E gyros, accelerometer ,encoders, motor drivers.
4) the forum is not as good. It is not well laid out and here there are people who know there stuff who are generous with their time. It doesn't have the same community.
I would personally disagree it's smaller 20,000 but that doesn't mean it's not as good, there are lots of people with over 4000 posts.
I also think it's laid out just fine, maybe better than the Ardunio forum.
I have a propellor I, the Propstick USB version. I work in cardiac electrophysiology and am no expert at microprocessors or electronics.
Well than you don't need a 8 core 32-bit processor.
A Arduino can't display 1080P video or make a full speed (12-Mbit/s) USB Host or have 8 simultaneous USARTs.