In general I think converting SPG to airlines mile is of poor value. Better used for hotel awards. If you really don't need hotel awards then keep them in SPG. When you need an award and star alliance is inconvenient or unavailable you will have SPG to transfer to the program of your choice.

So I think that SPG to UA ratio is not a great reason to not credit to UA. If UA is good for the other areas, then go with it.

Also why star. domestic redemptions I feel that AA is much better. I'd much rather have their miles. Personally my travel and goals sound similar to yours and I believe its not worth paying a lot to consolidate all into one FFP.

Although maybe you are asking where to put your miles for star flights only, and are already using other alliances too.