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Aroman, I've tried the R-55 a bunch of times and it is a great little combo. It's very vintage style with the open back and that great reverb, plus the high gain channel can get gnarly! But the Chubster is going to have deeper bottom end because of the cabinet it uses. A used R-55 goes for around $500, so they're a good deal, Chubsters are newer so even used they go for a bit more.
Also, I think the R-55 has adjustable send and return levels on the loop, the Chubster 40 didn't have that. I don't know if they've changed that on the new 55 watt Chubster.
Other than that, these (and most other current Rivera amps)are layed out similarly. Two channels, one based on Fender sounds and one based on Marshall sound, with a boost on each. The "Fender" channel has pull pots on the mid and treble for different frequencies.