Hey guys...
Lots of lurking on these forums in the past few years, not a lot of posting...but, having recently done a significant amount of research on these matters (with chief council interpretations, aviation lawyers, and AOPA), I'll add in whatever understanding or justification was provided for me:
14 C.F.R. 61.51(f)(2) states:
Holds the appropriate category, class, and instrument rating (if an instrument rating is required for the flight) for the aircraft being flown, and more than one pilot is required under the type certification of the aircraft or the regulations under which the flight is being conducted.
14 C.F.R. 91.109(b) in relevant part states:
(b) No person may operate a civil aircraft in simulated instrument flight unless—
(1) The other control seat is occupied by a safety pilot who possesses at least a private pilot certificate with category and class ratings appropriate to the aircraft being flown.
Thus, the safety pilot is considered a required flight crewmember for the "regulations under which the flight is conducted." (91.109 - Simulated Instrument Flight). Nowhere in 91.109 does it state that the safety pilot must be acting as PIC.
If the safety pilot was acting as PIC, then s/he would log time under 61.55(e)(iii), as they satisfy the relevant requirements for that regulation.
If the pilot manipulating the controls chooses to act as PIC, as defined in 14 C.F.R. 1.1, then the safety pilot would log the flight time during which they performed duties as a required flight crew member under 61.55(f)(2), or, as SIC.
There was a fantastic post in another aviation forum a few weeks ago on this very matter...I'll see if I can try to dig it up.
From a pragmatic standpoint, however, it's legal semantics. As long as you're properly qualified to act as PIC in that aircraft, I don't think it would particularly matter whether you log the time as SIC or PIC. The qualifications for acting as such are the same, and joint legal responsibility (not to mention insurance or legal liability) would jointly lie between the two crew members.