The 305 is over an inch wider. Otherwise the rest of the specs in regards to size are very close. This is due to the aspect ratio of the sidewall height being expressed as a percentage of the width. therefore the 40% aspect ratio of the 275 (4.33 inches sidewall height)is very close to the 35% of the 305 ( 4.20 sidewall)
The link below is very useful in comparing stock or any aftermarket tire sizes with each other. try it and you can see waht you can expect from the various size tires you plug in. Personally I'd go with the 305/35/20. Nitto also makes a 315/35/20 in the invo type tire. check them out on discount tire direct.com
U.S And Metric Tire Size Information Calculator
Nitto tires for car & light truck