Scott,

don't worry much about your filters as long as your new lens does not have a bigger diameter. You can always use a step-down ring.

All manufacturers have unique lenses with respect to different features. IMO, it absolutely makes sense to select lenses from different product lines according to your needs. No manufacturer offers all options from one hand. Coverage is one option, size and weight is another and usually contrary option.

I can comment neither on the 135mm Fujinon nor on the Nikkor. But the Rodenstock Apo-Sironar 5.6/135 is one of the best LF-Lenses I have ever seen. It is tack sharp even at f8 and therefore is able to deliver exceptional resolution. It has a nice bokeh and is of small size and weight. The Sironar-N is even smaller and lighter (but has less coverage). This might be an important aspect for field workers. The Rodenstock 135mm has got a 49mm filter thread. It might look a little odd to step down a filter from 67mm to 49mm, but you won’t see anything from that in your picture.