1- The Phenom X3 8750 is a old processor, so, is AM2+ and 65nm processor.
2- The Phenom X3 8750 is a 2.4GHz processor, the Athlon II X3 435 is a 2.9GHz processor, that with OC can get 3.5GHz or more
3- The Athlon II X3 435 is a 45nm processor, 95W processor and AM3 socket.
4- The Athlon II X3 435 is only $1,00 more expensive that the 8750.
Clock for clock Athlon II is slightly faster than Phenom I for most tasks, so since the Athlon II is clocked higher it'll perform better. You also get the benefits of 45nm for lower power consumption/heat on the Athlon II, DDR3 memory support, and higher overclocks.