As Doors to Automatic said, the 737-900 is longer. There are two main sub-variants: the - 900 and -900ER. The standard -900 model has the same fuel capacity, seat numbers and MTOW as the -800. The -900’s limited seat capacity is because it has the same emergency exit configuration as the -800 series. The -900s are powered by the CFM56-7B24 and -7B26 engines, with most having MTOWs of 174,000lbs while a few are as low as 164,500lbs.
The 737-900 was superseded in 2007 by the -900ER. This variant became a realistic competitor to the A321, while also filling a hole left by the 757-200. The overwing and Type I door exits were kept, but with the addition of two Type II exit doors, it was possible to increase the passenger capacity to a maximum of 215. With the addition of two auxiliary fuel tanks and winglets, the range is increased to 3,265nm. The landing gear, wingbox and keel beam structure have been strengthened to accommodate the increased MTOW of up to 187,700lbs. In addition there is a two-position tail skid and a flat rear-pressure bulkhead, which makes space for additional passenger seats.