The TW3500 utilises a 3 panel LCD system (one for each colour) for a more accurate colour pallette. Contrast ratio is a respectable 36,000:1.
The TW4400 also uses the same 3 panel LCD system, but uses the C2 Fine LCD panels for an improved picture. Zoom is motorised from the remote control (this is manual on the 3500), and it's two HDMI inputs are Deep Colour compatable. Contrast ratio is a far higher 130,000:1.
All Epson projectors come with a 3 year warranty.