Samsung SSD 830 (256GB) uses Samsung MCX controller, with NAND 27nm Toggle-Mode 1.1 MLC.
Samsung SSD 840 (250GB) uses Samsung MDX controller, with NAND 21nm Toggle-Mode 2.0 TLC.
Samsung SSD 840 Pro (256GB) uses Samsung MDX controller, with NAND 21nm Toggle-Mode MLC.
None of these techs are cheap, they are not equal, thus they performance differently, when it comes to sequential write, but if you consider sequential read, they performance about the same.
The 840 Pro uses MLC NAND, which is a lot better in write performance than the TLC NAND used by the 840, so in part the 830 is a bit better than the 840 to respond your 1st question.
As for your 2nd question, I would pay the extra $ 50 to get the Pro version.