I'm of the mindset of "Build what I can while I can", but I live in California where these weapons are demonized. So I've assembled a small collection of 11.5" to 18" ARs across a range of calibers.
While the 18" Mk12 and 18" Grendel builds are the ones I grab the most, my recent 14.7" 5.56 build has been impressive. Despite the handier length, it isn't that far off the performance of the longer tubes.
Since you already have the "goldilocks" rifle in your 14.5", I'd run it hard and figure out exactly how you want it kitted out before adding another to the stable. Once you settle in on the 14.5" role, you might find a performance gap where you want another rifle to fill in (like long-range hunting or benchrest shooting). Speaking from experience, an 18" barrel chambered in an intermediate cartridge (like .224 Valkyrie, 6mm ARC, 6.5 Grendel, or 6.8 SPC) can be a ton of fun at longer ranges.