I own both and would agree, the DX100 is better but not by a mile. I could live with the DX90 alone especially given its price as compared to the DX100.
Based on many comparisons the DX100 has better resolution,bigger soundstage and better bass impact and detail.If those statements are true then it is better.It doesn't need to be by a mile,after all we are talking about hi rez dedicated daps here and to some degree even that is subjective regarding sound quality.Guys like Currawong,Eke and Mimouille based on their tests don't believe the x5 sounds any better than an iphone as seen on the dap off thread.