At the moment the Electrum client is more secure as the wallets are password protected.

I am the lead developer on MultiBit and am working on wallet encryption but it will take me a few weeks yet. There are a number of changes to make.

I think multibit shines in its ability to handle multiple wallets, which is very important to maintain anonymity and information leakage through "change" transactions.

However, Electrum is currently a much more feature rich client that allows you to create wallets and spend bitcoins from an air-gapped computer with no internet connectivity. This is the ultimate security for Bitcoins.