If you only have one type of bike, by all means buy the other. I was N=1 for several years, until repeated mechanical issues were leaving me bikeless for days at a time. Went shopping for a proper N+1, and when I got ot the LBS I said, "I want to try the bike that's the least like what I have now." I ride my Ritchey Swiss Cross everywhere-- road centuries, climbs, fire roads, horse trails, you name it. I don't care how fast I'm going when I'm on it-- yesterday's dirt ride was 12mph average. It has a recorded aggregate average speed of 15.9mph over 7,250 miles. It's Cervelo stablemate goes fast. That's all it wants to do. 17.6mph average over 3,200 miles.

All I know is I could no longer manage with just one-- if you asked to make the choice, I couldn't. I need them both. So as a formerly dedicated N=1 guy, will 2018 be another N+1 year? I'm certainly not ruling it out.