The more natural sentence would be 'You're a real lifesaver!'. Between the two words of 'lifesaver' and 'saviour', lifesaver is mostly used.

For the first one, it's better to say "you are a lifesaver!", but the second one is correct. :) Either of those sentences sounds natural.

Hi Linda To say 'You are a lifesaver' is more natural. I would not use 'You are my saviour' as it leans towards the religious side

You could say "You're a lifesaver." "You are my savior." is also valid, but it sounds a little dramatic. As others have said, it has some religious overtones.

You are my lifesaver