If you have the SRAM 11-28 then your 39x28 will be approximately equivalent to a 34x25 (i.e. just one cog in the rear). Not dramatic. The biggest thing I notice between the two is how useful the 39 is at the upper end of my cassette. 39x14-13-12 is a lot more reasonable for rolling terrain than the 34x14-13-12.