Sidewall size... which makes a difference in overall height of the tire. If you're staying with the stock drivetrain for a while, the 195/50/15 is actually the best match to the stock tire in terms of overall diameter. You won't lose acceleration like the 195/55, you'll have better handling because of the shorter sidewall, and you'll actually put more rubber/tread on the ground when compared to the 195/55 too.
If you're going to swap to a B series engine, a 195/55 would be best for matched gearing. You can also go with the ever-popular 205/50 or 215/50 if you can find it.