Are you just going to go there to retire ? Or do you need to earn revenue and work ? That is the important question.
If you are looking for beach activity then Menton is really not the best place. It is very (very) quiet and family-oriented, especially off-season. The mild micro-climate attracts a lot of retired people, so it does not exactly bursts with activity, except a little in the summer. Also note that all beaches from Menton until Antibes are stone (pebbles), not sand! And Nice is not that close. The roads and traffic is such that you can expect a 1 hour drive to get to Nice.
If you are looking for a place to live on the french riviera, then you could consider Nice, or Antibes. Or even Cannes.
Poitiers is a nice little provincial town. Nothing much to do. Except the Futuroscope park. But it has quick TGV connections to Paris.