Actually Shiva means "Auspicious". Sada-shiva menans "ever-auspicious". both these terms belong to Sankarsana ie., Narashima.

if you have doubt regarding this then read 1008 names of Lord Narashima, there you will see Narashima is chanted as shiva, sada-shiva, etc...

Actually Narashima is the Sankarsana. Sankarsana is the destroyer. Rudra is devotee of Narashima. Rudra carries out his activities thru divine energy of "Sankarsana (Narashima)" only.

Vishnu & Narashima are forms of Same Supreme God - Narayana (krishna).

So we should not see any difference between Vishnu & shiva(narashima) as both are forms of Narayana(krishna).