Alright. Thank you all for the suggestions and recommendations! Let me share the result, maybe someone will find it useful.

I will add a little bit of more closure: this is going to be my second bike and on the 48 I got a very-very good deal.

I went back to the LBS to test ride the 50cm again. Now I'm absolutely sure that my perfect size would be between the two bike, but it's closer to the 50cm. I think I'd be able to have a very good fit with a stem somewhere 90/100mm and a zero setback seatpost.

BUT because I got a very sweet deal on the 48 and it's clear after trying the 50 that I can deal with the smaller size, I will stay with it for now. I will run a stem somewhere around 100-120mm and my current seatpost with 20mm setback seems to be suitable.

I'd be more torn between the two if it was my main bike, but that's not the case now. It was a super interesting experience, because originally I thought I would be between the 50 and 52.