If you want to get about and see a bit of the country I would say Sousse is your best bet, as you have both the main train station and the metro there, as well as buses and louages. Many people prefer Port El Kantaoui which is what I would call 'Tunisia light' (a purpose built resort with a lovely marina) but you would still have to get into Sousse to pick up transport to elsewhere, unless you want to rely on trips organised by your rep, which could work out expensive if there are quite a few of you.

Thanks alsemma for that info.
I will look into that.