I'm in a similar position as you, and have been reading and viewing mowers in this price range non stop the last few weeks. The SDX is on my short list.
One thing I can tell you is the 2 dealers I visited here in Louisville that carry the Hustler (along with other brands) did not seem to favor it over others they carried. Reliability issues in the 2016 models, that they thought were now fixed in the '17, seemed to shy them away from recommending it. They also didn't like the bolted deck. I'm not familiar enough with ZTR's to know how big a deal that is really.
One dealer offered me the SDX for $3900 + tax.
As for the Scag Liberty, everyone seems to recommend the Scag mowers, but I didn't' personally research that one. I was thinking it's almost 1000 $ more?