It kinda depends on the Pokemon itself. Rest Talk is good for bulky Pokemon like Suicune, Kyogre or Snorlax (to name a few), but it's obviously not good for fast sweepers like Haxorus or Mewtwo.
And Sleep Talk can pick Rest; you simply just have to take the risk that Sleep Talk might pick Rest. It's usually the case that Pokemon spam stat boosts like Calm Mind/ Curse/ Bulk Up and then use Rest so they can sponge hits for two turns while sleeping.