A traffic island has a raised area that can’t be driven over, whilst a mini roundabout has a raised area that can be driven over, if necessary. (I found this interesting
Any other difference between the two, shown below, is probably to do with road traffic legislation, which only a traffic engineer would deem significant. They cover near enough the same area and have the same number of feeder roads.
Although this mini-roundabout is probably less efficient, because one of its feeder roads has been blocked at the other end, to give Council employees a quiet life. Whilst the traffic island has become more efficient, because more cars are , we are told, being forced to use the ill-favoured access of Sherwood Way.