50-series were the sport, and 55-series were the no-cost option from volvo. you'd probably get better ride with 55-series (sticking with 205 width). so, if you want better ride, 55; if you want better road feel and handling, 50. as for z rated? depends how fast u wanna go. z rated tires have minimum 149 mph limit. h rated has max 130 mph, v at 149, w at 168, y at 186. not sure about your car but 98-00 s70/v70 series were electronically limited at 130 mph while 95 850 turbos were aerodynamic drag limited at roughly 147-150. the stock turbo were equiped with michelin pilot hx 205-50-16, and rated at w because volvo would find itself in a courtroom (which it later did anyway) if it had a car going faster than the tire was rated. h rated is plenty enough as 130 mph on public road is rather hazardous, unless of course u get on a track and may actually find yourself at the limit. in any case, it's not so much how much horsepower u have but how fast u are willing to go.