Every stereo company unfolds it new products at the ces show usually held in january and have those products out on the market around april-june.
My opinion is the 9851-55 are probably one of the best units alpine has ever made.
9851 is the normal swing series deck with the everyday functions built into and no glide touch.
The 53-55 add glidetouch, which in my opinion is a nice feature. If you have no patience and the the fingers of a gorilla, buy another deck.
The largest complaint of these decks is the they dont have any bass or treble just the preset eq curves.
But what they have is i-Personalize, which means you have access to I think either a 5 band or 7 band eq and all the crossover and time alignment processing you could want on alpine web site. This alowed alpine to focus on what they are good at. Sound Quality.
So if you want jumping dolphins and a race car and a deck that has crap for overall sq, buy a pioneer. If you want the best buy Alpine and use it how the manufacturer intended it to be used.
The 9853-55 are only decks for a complete system.