Blissey and Chansey have a very similar special defense; however Blissey gets leftovers. Chansey with eviolite has a LOT more defense than Blissey does, however. The third factor is that Chansey has too low of a Sp. A stat to use special moves, whereas blissey has enough Sp. A to fire off thunderbolts that scare gyarados, ice beams that scare dragons and flamethrowers that scare ferrothorn.
The general consensus is that Blissey is still better. However, if you're going to use a set which doesn't have any attacking moves besides seismic toss, chansey is an option. The last point is that Chansey also has less HP for things to leech with pain split/leech seed and stuff.