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in first the UI is different.
Within #gnusocial there are 2 themes. One looks like twitter, another a bit different.

Also in gnusocial you have the possiblity to create or join groups (this is until now used instead of federated hashtags)
create events ppl can say if they join or not, and run polls.
Also private Messages are much better sorted.
(not every instance have the mentioned function)
check out #postActive for the newest shit