For reference, your stock wheels and tires were very likely 275/50/20 because you have the luxury package. Those measure out to 31.99", so whatever is closest to that would be best. But anything around 33" or so I wouldn't bother concerning myself with how much the speedo is off. But having a programmer (SCT) properly adjust for the different tire sizes does make sure the speedo, mpg lie-o-meter, and shift points are all accurate. All four 275 sizes exist and are quite common, the others I'm not 100% positive about.

275/55/20 = 32.9"
275/60/20 = 33.0" cheapest size 33 for 20s
275/65/20 = 34.1"

275/70/18 = 33.16"
285/65/18 = 32.6"
285/70/18 = 33.7"