Markel and Sandro, which of them is more effective as a Necromancer?

Maybe.
There is still information about Markel besides his appearance in Heroes 5. It is known that he was previously the right-hand man of Queen Fiona, and some events related to this are mentioned in Clash of Heroes. Although his backstory is unlikely to reach the level of Sandro, Markel still remains a worthy character. And you can't call him an exact copy: if Sandro was a powerful and terrifying undead lord, then Markel is mainly a very cunning intriguer.