The train isn't very full in my experience, with quite a bit of room for bags, and is faster and you don't have to deal with traffic. It's very worthwhile. I think it's a bit busier in the early a.m. hours, but still not bad. It's common for people to take the train to downtown KL, and then take a taxi to the hotel.
They also have had some "mastercard" promos this year on the train, so you might see if they have anything like that when you arrive. I think I got 20% off for using a mastercard in January.
So UA offered to get you to REP, but it would have been a non-ideal routing? If that's the case, I don't think UA will be much use, since you are the one that chose not to accept their re-route. But I'm no expert on compensation, so maybe others have a different view.