I can't find the exact link, but on these boards a few players who went through both campaigns said that Tsar was overall harder and more dangerous than Rappan Athuk.
I found this interesting, in that of all the FGG products Rappan Athuk has the biggest reputation of being a PC meatgrinder.
To those who own/play both, is this an accurate estimate?
I'd say Rappan Athuk.
There's a *lot* of treasure in Slumbering Tsar (once they get to Tsar itself) and if the players make their own magic items (as mine are) using the rules as written, it gets much easier.
Also, the death ward spell (+ an oracle) makes several of the encounters that would otherwise be deadly extremely easy (in Tsar).
Plus, Rappan Athuk can be attempted with lower level characters (Tsar starts at level 7) and they die easier. ;)
My players are not through all of Tsar yet but I would also have to say RA and agree with DaveMage that there is a ton of treasure in the city.