I have had 3xH6 engines and none has ever required any oil between changes (4-7,500 mi, depending on conditions). However, I think I have read here about some who have seen some oil consumption, I am not sure if using synthetic rather than conventional may have something to do with it.
The H6 is Subaru's most reliable engine in the "current era," hands down (mid-90s to now). The current H6 debuted on the Tribeca in 2008. The current 2.5 debuted on the Foz in 2011 and the OB in 2013. It was supposedly revised for 2015, presumably at least in part in order to eliminate teething issues like the oil consumption some engines have. The current H4 should not have the head gasket issues that plagued the previous H4, particularly those of the late 90s, early 2000s. Still, we know more about the H6, obviously, than the revised H4 of the 2015.
Many people here have strong feelings about which engine is the better choice, with H6 owners typically steadfast behind the 3.6. I chose it in 2012 in part due to its known reliability and I have never wished I had chosen the H4. Now, the turbocharged H4 of the Forester is an H4 I would not hesitate to buy!
Long story short, if you want maximum value from your 2015 OB, the Premium H4 is probably your best bet. If you want the most capability, comfort, and reliability an OB can offer, certainly the Limited H6.