A bizarre choice between two extremes.
South Ken is very much international banker territory. Plenty of Americans, and also popular with the French, because of the Lycee. Lovely stucco houses mainly converted into flats, nice restaurants, expensive boutiques, good restaurants. Great if you have the budget and feel comfortable in monied expat circles.
Thamesmead has struggled in the past. Designed essentially as a new residential area along the Thames estuary, it failed to attract residents and ended up being mainly rental, often to people who could not afford anything else. Its improving. If you were a first time buyer with a limited budget it might now be worth looking at. Cross rail at Woolwich, the growth east along the Thames including the Olympic Park and the slow but steady regeneration of London docklands means Thamesmead is not as isolated as it was. So perhaps a canny long term investment. And I personally like the modern layout with lots of green. But there is not a lot there.
Honestly if you can afford South Ken, Thamesmead should not be on your list. If South Ken is a bit 'de trop', and I certainly found some of the competitive parenting a bit scary, there are lots of other choices. If you want something less urban, then Richmond, Wimbledon, Hampstead, and, if you are looking South East, Blackheath might fit the bill. Or if you want urban, Islington or Camden might work.