This is probably more of a request for an advice than a pinpoint question. Nevertheless I'm interested in hearing the take of others on the topic.
I have graduated in Germany, 2 years ago, with an excellent master's grade (converting to GPA: 4.0) in theoretical physics (c.f. Alleviating Dunning-Kruger effect without running into impostor syndrome) My work has no practical applications but I liked the theory and math. It took a while to get started looking for a PhD position, but I've been looking for one for ~ 1 1/2 yrs, first for publicly listed ones, then actively suggesting research topics myself. Unfortunately those who were interested in my suggestions had no funding available. Since my former adviser had no real external relations he could not suggest anybody to me to check for an open PhD position and I've been looking on my own.
Over the past months I have found one position in Austria where the topic fits to a T. It will be very mathematical and concern fundamental research (light-space-matter) with no immediate practical applications and possibly no immediate experimental verification (unless I can find collaborators to perform some experiments).
Simultaneously I got an offer for a PhD at a large research association in Germany (Helmholtz society) in a field I am not familiar, but am interested in. I got the offer after expanding my criteria though. This position would require a very nice mix of theoretical physics, mathematics and talking with experimentalists to compare theoretical predictions to experimental results.
The position in Austria fits perfectly to my interests, although I have (and always had) doubts as to whether I can handle the math. BUT funding is definitely available only for 1/2 year. Afterwards the continuation would be conditioned on a) the adviser being happy with my work and b) whether further funding will be available.
The position with Helmholtz does not fit perfectly to my physics interests. The funding and additional benefits are available for 3-4 years and I have no doubts that I can handle the topic. Its major "Pro" (apart from job security) is the active collaboration of different research fields, something I've been trying to foster during my M.Sc. (in vain I think).
I can not guarantee, that I will be able to supply the funds (self-fund a part of the PhD) should the funding run out, so my ideal position becomes quite a risky option. Given the assurance of availability of funds I would take that position without a second thought, but I very much want to avoid having to stress myself out after 6 mo. looking for a new position - again.
Is it advisable to choose a PhD position where the topic fits, but the financial security of the position is questionable? Or would it be more prudent to select the more secure position even at the cost of it not being a perfect fit? How easy/hard is it to get a post-doc position in a topic which is not perfectly aligned (still physics though) with the PhD one has done?
I might be able to secure some funding through an ongoing collaboration on a different topic (a pet project), but that will definitely not be 1 FTE (maybe 0.25-0.5 FTE).