Difference between RPC calls addmultisigaddress and importaddress
The difference between these two is that with addmultisigaddress, bitcoind stores the redeem script and the second one, bitcoind has no idea what the redeem script is and only stores its hash (which is essentially the p2sh address).

For watching the address, the two are essentially equivalent since you'll still know when bitcoins get sent to that p2sh address. The redeem script is necessary if you want to spend from the p2sh address to some other address. Even if you had the private keys in the multisig p2sh address, you wouldn't be able to spend it using importaddress since you still need the redeem script in order to be able to spend it.