Yeah I remember Chandhok doing the first 2 rounds of Season 1; then he left and I was really hoping he'd come back as there was a solid chance of points with him in the car. In the end I think one reason he left was a disagreement with the team, or with the team's management.
Now I get the feeling he is even better than he was then because of his experience in GP2, and also the team has changed quite a bit since the 1st series. I don't think there's a massive difference between him & Narain in terms of ability; instinct tells me Narain would probably be quicker over one lap but that Karun would give better technical feedback. The difference may be that Chandhok's career is still in the ascent to some degree; if he can stay another year in GP2 and do a good job he may get a role in F1 - either in testing or even a chance of a race seat. That would imply that for him A1 isnt currently that viable. Karthikeyan's career, on the other hand, has kind of levelled off since his time in F1 - and I think this is the kind of thing he enjoys and consequently considers somewhat more viable.
In a couple of seasons time though, who knows? We could have quite a pool of drivers looking to have a go in the team - including Karthikeyan and Chandhok - which would be pretty good to see.