Bulldogger: I was in exactly the same position as you a few months ago i.e. choice between a new TC40 & a used 2120. As others have pointed out, the 2120 is heavier. The TC40 uses the trans (the SSS, not the HSS) out of the 2120 (with an adapter plate) to match with the engine out of the 1920 series. Thus, the drive train has been around for awhile.
I ended up going with a new TC40 mainly because the price differnce equiped as desired (FEL and BH) between the two was less than $5k. An engine overhaul could have cost that. Also, the TC40 has some ease of repair advantages. For example, the hydraulic pump on the 2120 is in the transmission case while on the TC40 it is mounted externally on the engine (guess which is simpler to replace). Also, there are certain ease of access/service issues which favor the TC40 over the 2120. Visibility for some operations (especially loader work) is much better with the TC40. In other words, the TC40 is a newer design with the same basic, proven drive train as the 2120/1920 series. Unfortunately the TC40 has plastic type sheet metal, doesn't come with the nice metal toolbox behind the driver's seat (like the 2120), the 2120 has more weight in the chassis, etc., etc.
Probably would have gone with the 2120 myself if it didn't cost almost as much as a new TC40 (since with a new one, you get the break the engine in yourself, have full factory warranty, etc.). The price you stated on the 2120 seems good especially if in cherry condition (not banged up or abused). As others have mentioned, do a search for the 2120. Reports on this site by those using the 2120 for years are most favorable. Did you enquire why the thing was being sold with only 128 hours (practically new)?
JEH
PS The TC33 and smaller NH are not really directly comparable to the 1920/2120 series & TC35/40/45.