I will give you the 2 reasons why they are both good and then tell you what I would recommend for the specific roles
I will start with Snorlax.
Snorlax is a great guard spilt abuser because after a guard spilt, it's defences reach divine levels, coupled with it's amazing HP and good attack stat, Snorlax becomes very hard to deal with since it tanks almost everything whiles it hit back with stressing Power, especially with it's STAB move, return, Snorlax becomes almost impossible to stop. But there are so many threats to Snorlax, powerful fighting types can bring this big guy down when banded, life orb boosted, holding an expert belt etc... luckily there aren't many fighting types that can take on Snorlax. Beware of Rhyperior though because every snorlax that isn't holding seed bomb is doomed to fall to Rhyperior. But since it seems you might be using this ina multi-Pokémon battle more threats seem to come in and since both Snorlax and Shuckle are both very slow, this would be hard to pull of in a multi-pokemon battle since OU threats are not banned in multi-Pokémon battles.
It can be hard to pull of but when you succeed, it is pleasing
Snorlax is really a sweeper. With a raise in defence he can commence destroying teams without reall bothering much about health, be wary of status inducing moves though
Blissey is an already great wall that can tank and stall until you want to bang your head against the desk. Although Blissey is great, Chansey is usually preferred because of the eviolite that makes her defensive utility surpass that of Blissey's but they both play the same role. After a guard swap with Blissey, Blissey will reach supreme defensive utility and becomes a nearly unbreakable wall. Blissey has a very low attack stat moves but it has many other mobes that do fixed damage and work very well to slowly bring Pokémon down such as counter, good for (physical ), seismic toss (good for waring Pokémon down), sofboiled (good recovery move), toxic (slowly waring threats down till death) etc..
Again this can be a little hard to complete since Choice band terrakion's Close Combat destroys Blissey in one hit, so does Scizor with Superpower after one sword dance or with choice band but when you succeed it would be very pleasing as well
Blissey is a staller/tank/wall and does a great job at it. With a raise in defensive stats it can almost flawlessly complete it's duty
If you need a wall, I recommend Blissey, if you need a defensive sweeper, then I recommend Snorlax.
Personally I would pick Blissey since Blissey would be able to tank more hits
Hope this s of any help