I honestly don't think Kaiden and Carth are that similar. Carth is a vindictive prick who's extra pissed off about everything, where as Kaidan is mostly just...boring. He's the melba toast of the party's Chex Mix.
Like I know Raph Sbarge doesn't have a ton of vocal range but the two characters are pretty different.
I hadn't played KOTOR when I played ME1, but after I did I found them similar. The reason is because both are very private, professional people. They don't like to talk about their personal lives so trying to converse with them is like pulling teeth and what they do have to say isn't that interesting.