River Valley is typical urban, mixed buildings setting (older shophouses + newer
condos) and is adjacent to Orchard Rd area - the shopping Mecca of all the Singaporeans and tourists. River Valley itself, lots of small shops, some bigger malls, lots of coffeeshops, a few more fancy restaurant. Not much more.
Tanglin less urbanized, lots of green areas, many landed houses and low rise buildings, embassies, not so many shopping centers but one pretty good at the begining of Tangling Rd. Close (adjacent) to Botanic Gardens. Inside Tanglin is Dempsey area - restaurants (regular and more fancy), wineries and antique shops mostly, all in pretty relaxing setting (perhaps except Friday nights).
Transportation-wise, River Valley probably better but there may not be any practical difference.
Use google street view to get a better idea
Edited: River Valley, not Riverside.