As of now, the new 2019 Forester and the 2019 Outback are pretty much identical in most aspects of the car, ie., tech, longevity, handling(to a point), comfort. Two of the most prominent differences between these two models are the following:
1) Forester is built on the new Subaru Global Platform(SGP). What this means is that the frame is built better, is stronger and lighter than previous generations and prior platforms. All Subaru models are slowly being transitioned to the use of the new SGP; 2020 Outback will build with the SGP but the 2019 Outback models are still stuck with the old platform.
2) The Outback models have an option for a 3.6L engine whereas the 2019 Forester maintains the use of the proven 2.5.
Other than those two differences, the choice really comes down to your personal wants. I did consider the 2019 Outback but I was drawn to the 2019 Sport after multiple test drives. The Forester, Sport model in my case, just felt more nimble and responsive compared against the two. The Outback does weigh more than the Forester I believe and that may have been what I was feeling. But they both were excellent choices to start with.
If considering 2018 Forester models, keep in mind that a turbo variant is available. However, it is not based on the SGP and it is a little smaller in dimensions than the newer 2019. Also, all 2019 Subaru models come standard with EyeSight and other safety tech that would typically cost extra with another car manufacture.