I see no reason to pickup the XDS 9 if you already have a XDS 45 unless you just like collecting pistols and have the latest model out there. You can carry the extended 7 round mag in the XDS 45 with minimal extra bulk if you want more rounds.
I'm getting one nice to have options! I own a xds45 and xd9 and xd40. They all have their good and bad qualities and I assume the xds9 will be the same, but buying these guns can get addicting too!!