Of course Multibit is safer.
Bitcoin Core > 3rd party Bitcoin clients > online wallets
As you can see, online wallets are the worst decision for storing Bitcoin.
Your comparison is wrong!
Bitcoin Core = Other clients in matter of safety.
No, of course not. It's impossible to put Bitcoin Core with other 3rd party clients to equality. Even they are open source you can't %100 trust them as you do yourself.
Even though I agreed that Bitcoin Core is more secure, I would have to say that your point is not solid. Bitcoin Core can easily be changed just like any other SPV clients, the developers can easily push an update to change some of the things. Opensourced doesn't really help unless you review the source code yourself and compile it. If you do, any client should be as safe as Bitcoin Core.