Gsr would be a little bit more reliable to use, but the b20v you will get more torque thing is it will not last long if you dont have the block built with aftermarket pistons and rods reason why is because the stock rods on the b20 are only made to be taken up to about 6300 rpms, when you have a vtec head you are going to need a vtec ecu and most vtec ecu's red line at about 8,000 rpms... so in your case i would just go with the gsr but if you have money to spend the b20v is nice. about the nitrous i would stay away from it unless you have money to keep buying new motors or get one built just for nitrous... turbo is the best way to go! goodluck