I just recently got a Monochrom and am a long-time user of Aperture and the NIK Software plug-ins. So, I have been using Silver Efex Pro 2 (SEP2) for all my processing. I first use 'Boost' in RAW Fine Tuning in Aperture to recover highlights, and then go directly into SEP2. I haven't done any processing using just Aperture because I like what SEP2 does and the controls within that application are great!
I tried using Lightroom 5, but couldn't make it work for me--I've been using Aperture since version 1 and am very comfortable with it, plus I like the way Aperture integrates with other applications, and the file management in Aperture is amazing!
Good luck... Look forward to seeing what you produce!!!
Cheers,
Michael