On one platform? 3090 and 3310 have different ancestors, as an quick overview: A9500--->A9800--->S3988 --->PMW3310 lots of A30xx´s(except 3050)--->A3090--->EOL A5050--->A3050/55--->already EOL or soon AM010--->PMW3320
Maybe at one point prior to the patent trade it was handled as such but i dont have any info on that and DA 4G release and everything are overlapping there, but even if 3988 would have been public from the start, 3310 would have been a "necessity" considering casing and price, considering as of what is now, they just fill different roles.
From a consumer Standpoint, Variance. From an Industry Standpoint the cheap price and low power consumption. It would actually be interesting to know how many people who are talking about 3050 have ever tested one, im not a personal Fan of it, but i probably would prefer it over 3310 if you would only give me those two options, but then again im not a fan of 3310 either.