The sidewall of a 205/50 tire is only marginally shorter than a 195/55 tire.

The "series" of the tire (50, 55, whatever) is the sidewall height expressed as a percentage of the section with (205, 195, etc). So a 205/50-15 tire is 205 mm wide, and its sidewall is 102.5 mm thick, while a 195/55 tire is 195 mm wide and has a sidewall thickness of 107.25 mm.

That said, I'd still go with the 205/50-15 because it is a bit of an improvement.

This is a good resource for comparing overall diameters of different tire sizes: http://www.miata.net/garage/tirecalc.html