Chansey isn't that much more specially defensive than Blissey, only bulkier on the physical side (significantly so). However, this comes at the trade-off that Chansey can only really use Seismic Toss as an attacking move, as 35 SpA doesn't let it use the moves Blissey commonly uses (Flamethrower, Ice Beam, etc.). It also loses on Lefties recovery. IMO it's not worth it to use Chansey over Blissey.