[HECnet] RSX or RSTS

With all this social distancing and staying home, I've been dusting off old hardware and fixing it up. I just finished running diagnostics on a nice little PDP-11 system. It is a - PDP-11/53+ with 3.5Mb RAM in a BA123 KDA50 and 2xRA73 drives DELQA NI CXA16 16 line mux (DHV11 clone, more or less) TQK70 and TK70 (probably worthless - those things never work) SQ739 SCSI controller (does both MSCP and TMSCP!) I figure I'll use the SCSI for a CDROM and a tape drive, maybe an Exabyte. Don't really need more disk space with two RA73s already, and I'm sure the SDI is faster than the SCSI. So, the question is, what OS should I put on it? RSX or RSTS? It has to be something that can make good use of the hardware. No Unix, please - I've already got a nice 11/83 with 2.11bsd on it. Ideas? Bob