Starting around soi 49, the expat area starts to converge more along Sukhumvit Road, down to about the first kilometer away. The further down these sois you go, the more rustic Thai they become, starting with soi 49. The expat highrise condo region ends shortly after Sukhumvit 63, though some decent houses can be found in some places down past soi 63 in zone D.
On the south side of upper Sukhumvit, the expat region pretty much ends at Sukhumvit soi 28 and gives way to a crowded Thai residential area, though there are some scattered houses and condominium properties that conform to expat standards. I expect this area beyond soi 28 to see substantial development in the future, though that seems to be only in its infancy.
Choice of residential location usually depends on school location for any children, work location, means of transport (rail, car, walking), and traffic flow if driving or taking a taxi. These and other issues are best address on a region by region basis as covered on our sister website www.ssbkk.com, which for each station has points of interest, a decription of the ambiance and elements of the region, and a list of properties in that region as shared with our property website, www.kkBkk.com .