If a post-graduate student has three unpublished studies, one of which is expected to have a high impact index, and the rest low impact, what should the student reserve his/her best work for, in order to get best recognizement: either his/her monography, or a high impact symposium, or a high impact journal?
Get your best work published in the best journal you can manage. The "best" journal will depend completely not just on your field but on your speciality and the school of thought you subscribe to. Ask your colleagues if you're not sure.
Also apply to present your paper (published or not) at the best conferences you can get accepted to that you can manage to go to (again, "best" will differ depending on the specifics of your work).
You do not have to choose if your paper gets published or gets a talk. Do both. You can also do multiple talks. If you can, get all 3 studies published (either as 3 separate papers, or as 1 or 2 papers, depending on how related they are and how your field handles it). Even if they are 3 separate papers, it might make sense to put them together for your conference presentations (again, ask people in your field for guidance, as this varies a lot).
Remember too that, if there is a different in best journals/conferences between where you got your degree and where you want to get a job, aim for the latter. Conferences are the best place to find contacts that will get you post-docs and other jobs. Good luck!
It is difficult to predict from where you - or any post-graduate student - might gain recognition for unpublished work, perhaps it might be useful to consider instead where the work might quite easily be published. The 'linkedin' website for professionals offers the facility to publish articles online that can be shared widely, or just to connections.
The style of write-up of studies, with findings, could be adapted to suit a larger audience without losing integrity, or confined to a chosen scientific group. I suppose whether or not this method is suitable depends on whether the concern to protect authorship of findings (quite understandable) is of more importance to the individual than the sharing of the information.
My writing on that platform differs in that I used a combination of personal professional experience and a literature search of related topics, and my absolute aim was to share information.