I look in on both Phoenix rising and Science for ME forums regularly. I'm spending more time on the other at the moment as there are interesting developments on PACE and refuting BPS front and good detective work being done that will in my opinion make a difference. The sciencey discussions, which have a lot of contributions, can be over my head, but I've found it ok to ask 'stupid questions'.
I don't see a difference in support if you need it.
Some of us can't get on the other site even after asking for and being promised help.
This sounds weird. How did you ask for and were promised help if you can't get on there?
Phoenix Rising has a bigger depository of useful info on it's site that it's members have accumulated over the years. I'd say there are knowledgeable people on both.
I don't know anything about other forums.
I’ve no idea what “other” site vies for members ?
It's not a competition, there's no problem belonging to either or both if that's what anyone wants to do. There are other forums too, I think, I just don't know anything about them.
edit to add - I was forgetting Health Rising, so that's 3 I know about. Also edit for clarity