Within the Encore line (Alto/Strato/Terra), there is only one Terra server, but several models based on storage capability (currently 6TB, 22TB, 48TB, 88TB). They should all download at approx. the same speed. The discontinued Terra servers (12TB, 18TB, 24TB, 40TB and 72TB) all functioned the same, and downloaded at the higher speeds (compared to Strato).
The Strato Players have two models, a Strato with internal storage (Strato, Strato-S), and a Strato Player used for playback only, no internal storage (Strato-C). You either need a single Strato w/storage (Strato or Strato-S), or a Terra server and Strato-C Player, to download and playback content (including 4K) in the Store.
Obviously download speeds are dictated by many factors in an owner's system, but you should find the speed differences with the K components is actually between Strato and Terra, not between old and new Terra servers.
The point is, any speed differences between Terra's should be negligible, and not even something I would consider when shopping for Terra.
Jim