What are the 'objective circumstances' that one young researcher should take into account when choosing a research topic?
There is no "optimal coice". Getting "good" results, and recommendations does certainly not only depend on the choice of the topic, but more on yourself and your motivation. And that is the point. If all possibilities seem to be equal I would apply for the group in which I feel most welcome. It is so important to be in an environment in which you are allowed and encouraged to fail often (and early); where people leave titles at the door; where you can build on ideas of others and people dare to be wild and have fun !

Forget the results and, the recommendation afterwards. Do what you like to do, together with people you like and the results will be good automatically. You will find opportunities for future projects you never thought of yet. And to be honest, I do not think an undergraduate research project will change your life. Explore your style of work, your weak and strong sides and be open for ideas. I would not expect too much at this stage ("in terms of mathematical results achieved").

OK, that was a statement based on my personal opinion, but it is based on my personal experience as a PhD.