First and foremost Linus was a MINIX user which was based on UNIX V7. That could explain his preference for SysV.
Andy - please explain that connection, as it seem a tad tenuous to me. Sorry for my thickness, but I don't see how being based on V7 reflected on being System V based or not. System V was released many years after Seventh edition (1984 vs 1979). BSD had been out for years by the time Linus started his work and most everything from Seventh Edition (ney UNIX/TS) has been taken into PWB 3.0 @ Summit which was renamed System III in 1981/82ish by the North Carolina weenies at AT&T.
I'm trying to think of a program I wrote for Seventh Edition that other than compiler and architectural differences, would not run with a "make clean; make" incantation (I can think of a few but not many). I guess the primary differences was init(8) had changed at that point, /etc/rc had been restructured, and the terminal handlers (termio vs {g,s}tty); but from an kernel interface standard point except for the terminal handler, System III was pretty much based on Seventh Edition and System V was a superset of that.
If I remember a number of the emails/notes from the time, Linus was using SunOS on Sun3 at school. So he seen all of the BSD extensions to Seventh Edition, and in particular networking and the sockets API. As Ted noted, POSIX had more of an influence since it was at least formally described, while BSD was only defined by 'trusting the source.' POSIX had not yet been able to broach the Networking API at this point, but things like termios had begun to appear. In fact, Bostic was in the process of redoing BSD's terminal handler to add that API IIRC [I can ask him of the actually date].