one which has a huge dome filled with the ashes of the victims, and that it's extremely quiet in terms of visitation
That's Majdanek, in Lublin. Probably the busiest of the AR camps (due to its location in a city).
Sobibor is completely different, by far the most 'original' of the AR camps. I'd recommend a visit soon if you're going, there's a plan to dig half of it up and build a museum in the same way that Belzec has been redeveloped. Belzec is well worth a visit but more on the emotional level that on the historical level.