To summarize:
SFP = option to send 1 Gbit over copper OR fiber. Longer distances or between buildings = fiber. Shorter distances and inside the building = copper (and you can just use a normal RJ45 port in that case. You also need to be careful because often the SFP ports are shared with a normal RJ45 port, meaning you lose one RJ45 port when using SFP.
SFP+ = option to send 10 Gbit by default over the media mentioned above in the cases mentioned above.
How you do it structurally, i.e. a central 10 GbE (core) switch to which the other two switches and e.g. a NAS with 10 GbE are connected, or simply both switches connected to each other with 10 GbE, can be considered and decided when you know what you want to use.
IF I were building new anyway, I would avoid creating a bottleneck between the switches and directly use SFP+ with fiber or copper. No one knows what you might want to do later. Maybe you want to use MoIP someday and then you’re stuck. There are many applications. In the past, people also thought that 64 kbit ISDN internet was enough for everything. My work computer has a 10 GbE card and is connected directly to the core switch because I almost exclusively work with data on the NAS. But everyone does it according to their taste.