The 25mm lens is 72 degrees of coverage, the 28 is 65 degrees. For me, that's a sizeable difference that would warrant a matching finder. On the other hand, all accessory finders are approximate, offering at best parallax correction lines, but not expanding or contracting frame lines as focus changes (and with the change in focus point, a slight but noticeable change in lens focal length). A rangefinder camera's built-in viewfinder is better for correcting these errors, but far from perfect. The real question, as I see it, is what your style of shooting is like. If it's fast, and approximate, you might have no need for the 28mm finder. A more studied approach, with a need for more accurate framing, might call for the matching finder. Really, only you can decide!
I love shooting with rangefinders. But when I really need precise framing, I'll switch to an SLR with a 100% viewfinder. Perhaps heretical for devoted rangefinder shooters, but, as they say, the right tool for the job.