This is a decision You have to take on Your own in the end.
Let me add a few points though for Your consideration.
- Both contracts are limited to 2 years at the moment. OS (definitely) but presumably also EW are interested in retaining as many of You as practicable afterwards, but 2 years is a long time with regards to the current situation of LH and associated companies.
- You are a NFF and obviously and understandably interested in joining LH. This may or may not become possible again and is subject to what VC and the GL come up with in their (currently halted as far as I know) negotiations.
- The moral objections against joining Eurowings are shared by many, not only in LH, but also in other companies.
Those points made, let me play advocate of the devil and speak in favour of the DH8 with Austrian.
The Dash is an aircraft of a category You will likely never see again in Your career. It comes with operational peculiarities that are well worth seeing and able to add a bit to Your flying experience. A network ranging from large airports like FRA to small mountain airfields like Lugano, in an aircraft limited to FL250 and offering rather little assistence to its crews compared to an Airbus brings some challenges that are well worth seeing for some time in a career. In fact, the rather many NFF we have on this fleet at the moment seem to like both the aircraft and the fleet quite a lot.
With regards to the career cul-de-sac that the DH8 is apparently considered by several around here: this is in fact not the case at Austrian; neither is it the case, should it become possible for You to join LH afterwards. Several DH8 F/Os, among them some NFFs with a bit less than 2 years of service on the DH8, are at the moment sitting in the ERJ 190-200 type rating class. Applications for other fleets are honoured only depending on seniority, not depending on the type previously flown, and the success rate of those type ratings gives no reason at all to change this. Most pilots who have flown both a jet and a turboprop will agree that the (any!) jet they are trained on is much easier to fly and operate than the turboprop, and there is no reason to believe that anyone who has mastered the DH8 will have problems mastering any jet.