Neither one is better, both have advantages and disadvantages. The 350 has a lower first gear and will produce less of a drop in engine rpm's at each shift. But the PG is lighter and "eats" less HP to turn, which somewhat makes up for this. When set up with high stall torque converters and low rear gears PG's have proven to be very effective on the dragstrip for years. So effective in fact that adaptors are available to mate the PG to other make engines besides GM. The PG also works best in a light weight car, which the Nova is. (especially the early ones)
With a heavier car and/or a milder engine, the 350 would be slightly quicker and more fun in my opinion. But if you set up a PG as stated above with a high revving engine you will not be disappointed. Plus smoking other cars and then explaining you only have a 2 speed is pretty cool in my book.