Shane (in my opinion) has done the worst thing so far, by telling her, Rick was dead when he wasn't. Even though "WE" know that he had no hope in hell of getting Rick out of the hospital.
Rick did some equally (less in my opinion) bad things, like kill Shane and not tell the group that they were infected.
In all, they had their reasons but these aren't obvious to Lori, only the viewer.
Also, think about how both men have treated her....
Shane has protected her 100% from the start, and trust me, if it was happening here and it came down to sacrificing any of my friends or neighbours to save my wife and kids, I would do it in a heartbeat.
Trust me, this goes a long way with women
Rick however, since he got back, what has he done? buggered off at every single opportunity, not just putting himself at risk but also not caring what happens to his wife and son whilst he is away for simple things like using what ammo he has left, to get a little bit more ammo, just to give it away? what the hell is up with that?
As a viewer, I didn't like Shane but it has to be said, he was a better man and was the only realistic person out of the bunch, who had his feet firmly on the ground and new what was important and what wasn't.
The writers did a good job of getting it across that Lori also had the same opinion