None taken; by wrong I mean you probably have some form of personal attachment to the GT25xx that causes you to feel it's superior to the X5xx. Don't think that's a knock on the Cub Cadet, so much as a testament to the overall quality of the Deere. Mechanically, the GT25xx is a solid machine; the Kohler Command is an excellent engine, the Hydro-Gear rear is absolutely solid, and these basics make up the current Cub Cadet XT3- which I've owned previously. The shaft-drive, a hallmark of Cub Cadet, does an excellent job maintaining a consistent transfer of power throughout the life of the machine, where the efficiency of a drive-belt will ware down in time. That aside, the design of the hood was absolutely preposterous, and it's ability to trap heat was second-to-none.
Now the Deere X5xx runs Kawasaki FH engines, to include formerly the liquid-cooled X540 and current fuel-injected X590; a belt transfers that power to a TuffTorq K72 rear-end, which provides more torque than the HG BDU-10 in the Cub Cadet. All models of the X5xx include a differential lock, I don't believe the GT25xx offered one, though eventually Cub offered it with the XT3 GSX- two generations after the GT25xx. I don't recall if the GT25xx offered hydraulics, or if you had to bump up to a GT30xx to get them; the higher-end X5xx machines had a basic hydraulic system that provides both power lift and steering- and I believe also powered attachments like the sleeve hitch. [EDIT]: Also, the Deere machines are offered with 4-wheel steer.
Both machines offered belt-driven rototillers, were excellent snow fighters, and had their fans. Both are, for all intents and purposes, still in production- though it's technically been two generations since the GT25xx was new, the XT3 is a direct descendent.
The Cub Cadet offers productivity and features similar to Deere at a lower price, where the Deere has (arguably I suppose) superior ergonomics and absolutely superior fit and finish. The Deere is also physically larger, and heavier.
I've never owned an X5xx, and absolutely loved my XT3, but there is a reason the X5xx sells so well- and it's not because they're a bargain.