So over which type, I mean the full descriptive name, of digital connexion can I connect the USBPre2 to the PMD661 on the one side and the D50 on the other?
Jon (mshilarious) was just making the point that S/PDIF is a data format, and can be carried via a coax cable (usually with an RCA connector, but sometimes with a BNC connector) or an optical cable. but it's the same S/PDIF format either way.
The Sony PCM-D50 provides an optical S/PDIF input.
The Marantz PMD-661 provides an RCA jack for the coax S/PDIF input.
The USBPre2, conveniently, provides an S/PDIF output over both optical or coax cable. So you'll be able to connect either deck directly to the output of the USBpre2.
I generally prefer a coax cable for S/PDIF signals, particlarly for field recording, as I think they are more durable and reliable. but, I don't have direct experience with the PCM-D50. If the optical S/PDIF connection is tight and secure, it probably won't make much real world difference which one you go with. Perhaps other D50 users can speak to the optical input's reliability and durability. That said, the PMD-661 provides an RCA jack for the S/PDIF input, and it's tight and secure. the cable can rotate and/or shift a bit, and there's no worry about loss of signal. and there are many right-angle RCA plugs available, which I find convenient.