Here's a video from European professional, Mark Crossfield, showing the differences between Ping's S55 and i25 iron models.
I plan on trying both of these model of irons from Ping sometime this year as I am interested in irons with more of a bounce angle as the Florida turf does not interact that well with low bounce angle clubs.
While I have traditionally favored a blade design, I feel that today's irons there is not a lot of difference in the MOI around the sweetspot in irons outside of the 'game improvement' models. I don't need a lot of forgiveness with irons, particularly since I utilize MOI matching. But, what tends to be more important (IMO) is the CoG placement. Crossfield's video shows the importance here as he is obviously a low handicapper with a good swing. That would make one think that he is best suited for the S55, but the i25 model ends up being the better fit.