Yes

Could someone tell me what's so great about the AA..

AA is where all the rich rich kids go, you'll make loads of contact but RCA is where all the talent kids go imo

Both are good schools. I don't have much direct knowledge of the RCA but the feeling I got while I was in London was that it was at its best while Nigel Coates was there, much the same as when Alvin Boyarsky was at the AA. 

AA gives you a lot of freedom to set and follow your own agenda, and there's a lot of variety in the unit choices available. You should already have a good idea of what you want to do at the AA before you enter diploma (there are of course other MA courses available, but the diploma is at the core). If you just come because you've heard it's "good" you might find yourself lost after a few months. After which you have to grind and grind and grind to get a project good enough for the tables.

Neither of the schools are going to make you into Rem och Zaha, though. That's entirely up to you.

I'm actually leaning towards to RCA because the opportunity to collaborate with people from other design/art disciplines is attractive to me, or more like I somehow cannot imagine being in an environment where everyone's an 'architect', I enjoy architecture thinking and learning but I'm not 100% restricting myself to become a qualified architect. I don't know the best way to put it, but I also want to be making the right decision. To be fair, I am always more interested in architects/designers who graduated from the RCA. I really like their sense of aesthetics and presentation styles as well.

Both AA and RCA have units that interest me and both seem to allow my multi-disciplinary approach to sprout. I wonder if it would be silly for me to accept the offer at RCA rather AA's. I also heard AA's course is really intense and may fail/redo a year easily, is that true?

It seems like many people would compare the AA with the Bartlett, I don't seem to hear as many strong views on RCA Architecture. Another thing to consider is that, I am a UK home student, RCA would cost me £9750 per year whilst AA would be more than a double (£22000), I just wonder if the doubled amount of tuition fee is worth it at all, as many people tend to glorify how great the AA is, I'd like to know more.

Hmm.....since this is your 3rd or 4th thread on your this/related topic perhaps you should take a year off and find out what you want to do with your life.  If you like the RCA go for it.  If you like the AA go with that. Don't look for us to give you that silver bullet to go left or right.