I'd say it really depends on what types of attractions and experiences you enjoy most.
Richmond (my home town) is a beautiful little village built in the gorgeous Georgian style, and it is fantastic for a gentle walking exploration of its main street and historical sites, including the famous Richmond Bridge (oldest bridge in Australia still in use). Bonorong is the best place in Tasmania to experience our native wildlife as well, and they are also a wonderful organisation to support due to the rehabilitation work they do for sick and injured animals. Mt Wellington is an absolute stunner, and would have to be one of the most scenic views of a capital city to be found anywhere in the world (no joke!). You may even get snow on top.
On the other hand, Port Arthur is also beautiful and has a rich, sad history that all visitors to Tasmania should experience (your ticket will likely include a cruise past the Isle of the Dead as well). It is an unforgettable site that will definitely make an impact on you. There are many other fantastic natural attractions on the Tasman Peninsula that you may visit, but without knowing your itinerary it's hard to say. I suspect it won't be too many as it is a 1.5hr drive to get to Port Arthur from Hobart and another 1.5hr return (whereas Richmond is only 20-25mins from Hobart). The wildlife park on the Tasman Peninsula (if you visit) probably isn't quite as good as Bonorong based on the reports I receive from my community over at the Lap of Tasmania, but you will still have a fantastic experience seeing Tassie Devils.
Both will be fantastic though, so don't stress about making the 'wrong' choice 🙂
If you have any other questions please don't hesitate to ask!
Andrew.