Hi, Naomes -
I cant comment on the Gaggia, but I can say that we have been very pleased with our Rocky/Silvia combo. I added PID control right away (which is contrary to the advice of many who believe you should spend time learning the machine first).
I chose Silvia over Gaggia primarily because the Silvia has a brass boiler. I believe that the Gaggias use aluminum. Whether that was a reasonable basis for making the decision, I have no idea!
I also became convinced that the Silvia has enough thermal mass to give stable temperatures during a shot (I have found this to be true).
Biggest disadvantage we have experienced, as with any single boiler machine, is the wait time between the steps in the "brew / steam / cooldown / brew" cycle when making multiple lattes. Im not sure you can solve this problem inexpensively.
Hope this helps a little. Now I will wait to read the thoughts of the real CS experts :-)