Thanks for the input thus far, folks. The bike is a 1979 Mercian so it would have been built up to the original customer's specifications, and I received it as a bare frame set so I can really do whatever I please! Both the 999s and the 605s would have been made that year, according to my research. I'll have to configure the frame a bit to make sure the 605s will actually reach around the fenders I might be using, now that I think about it.
I'm glad to know that some folks don't think the difference in general performance is very drastic. For whatever reason I feel as though I'm having to unlearn something that I gathered from spending time on this forum, in fact... But perhaps the issue with side-pulls was with some of the Weinmann models in particular, and not the technology overall.
I'll definitely be using Kool-Stop brake pads with either choice! That's a given for me these days. Cheers!
-Gregory