As far as I can tell, there doesn't seem to be any difference. Neither have wifi capability as well as Ethernet. Depending on what you need them for, do you need the high speeds? It will only be of benefit within the LAN for streaming from a NAS for example, otherwise your broadband speed will be the governing factor.
I can confirm that there is no difference in the Spec of the BT Mini Connector and BT Extender Flex 1000.
Primarily, the BT Mini Connector are promoted/offered to BT TV subscribers where the customer's Hub in not in the same room as the BT TV STB. The only difference (apart from cosmetic) is that the BT Mini Connector's Ethernet LED doesn't blink when data is being transmitted. This was a result of feedback that the blinking LED can distract when postitioned in a socket near the TV.