I was. Im not aware of a 12-28 with a 18t cog. The DA one replaces the 11t with a 16t, but no 18t. Reason Im asking about this and try to extract some more specific info from OP is, it may be the 18t he misses, or something else that is the same between the 12-28 and 11-28 anyway. The SRAM 11-28 that OP is using, already has a 16t, but jumps from 19t to 22t, where as the the Shimano 11-28 is 19t-21t, but loses the 16t. If he gets the DA 12-28 he retains the 16t and gets 19t-21t instead of 19t-22t or he could get a the shimano 11-28 and get 19t-21t, but loose the 16t. The DA cassette is pretty expensive though.

http://www.ritzelrechner.de/?GR=DERS...&SL=2.3&UN=KMH

http://www.ritzelrechner.de/?GR=DERS...&SL=2.3&UN=KMH

http://www.ritzelrechner.de/?GR=DERS...&SL=2.3&UN=KMH

Me? I would get the 11-32 to gain one more low gear from the big ring.

http://www.ritzelrechner.de/?GR=DERS...25,28&UF2=2135