the 8600 mp isn't the older version of that deck the 860 is the higher end premier version and the 8600 is the lower end budget version and yes they are great decks with a ton of features
I'd just go with the RB20 since it's more of a deal than the RB10. Oh yeah, watch out for the "Similar to RB10" or "Generic RB10" ones on ebay, those require an XM tuner to work properly.