The sizes you are looking at has the following specs:coastietom said:
275/55-20: width=10.82in sidewall ht=5.95in tire diameter=31.9in
305/50-20: width=12.00in sidewall ht=6.00in tire diameter=32.0in
As you can see, the lower profile (50 series) tire is actually taller than the 55 series tire. The 305mm of course is wider than the 275mm and has more tire in contact with the road. I have a 305/50-20 tire and I am happy with it. Click on the links in my SIG for photos if interested.
To compare sizes, check this calculator: http://www.1010tires.com/TireSizeCalculator.asp
Hope this helps