I know they are very similar but the steps at the top are a bit different and I don't have much experience with these things. I have the 12-27 now and need a new cassette for a new set of wheels. I don't need anything smaller than 12 as I almost never use the 12 anyway. But I do use the 27 on climbs and a 28 would be just a tab bit better. However, what I mostly use are the 15-24 with the front 50. That is where I end up doing 90% of my pedaling and there doesn't seem to be a big difference between the two.

I feel I do end up cross-chaining a lot as the upper 3 cogs tend to be ideal on small-ish hills and it's easier to do that than shift the front. the 12-28 gives just slightly different cogs there and my help or hurt some. Is gaining the 28 worth the change from 24-27 to 23-28? I don't know why they made that second cog smaller on the 12-28 (a 5 tooth jump instead of 3).

Also, does a 28 fit a short cage Rival derailleur? I've heard both yes and no and I don't see it on the specs on Sram's website. Thanks.