They are all made by MTD, it is Cub Cadet's parent company. But the 2500's and 3000's are their flagships and are sturdy, well built machines. The 2500 series weigh in at nearly 800 lbs with their decks, which shows they still contain a lot of that magical stuff we used to know as metal.
I can only think of one 2542 owner who posts on here regularly, who wishes he opted for the 2544- the 44" deck is really outstanding compared to the 42", that's why it's so highly recommended. But I have not seen any real complaints about the 2542, so if you like it I wouldn't hesitate to get it. Short of a 1 HP rating difference on the engines, (and the decks) the 2542 and 2544 tractors themselves are totally identical, so just about all the stuff you can read here about the 2544 pertains to all the 2500 series Cub tractors.
-Fordlords-