If you're gonna switch to FreeBSD anyway why not just a generic 19" x86 server ?
Price, and reliability. Dell rackmount servers hold up fine, but they're way overpriced. As for the generic 'built for linux' type servers, we've tried a few, and had way too many problems with them. (We got some machines from Penguin per the recommendation of another site involved on a project ... of the 4, two were RMA'd ... one had to be sent back a second time).
As I'm a federal facility, RMAs can add a week to however long it normally takes ... gotta blank the drives, fill out the paperwork to get the item untagged, fill out the shipping paperwork (even when freight's paid for, gotta declare what's going out), take it to the shipping warehouse ... wait ... wait for shipping and receiving to x-ray the returned item & tag it ... wait for shipping to deliver to our building (and they only deliver on Tuesdays & Thursdays for our building, due to staffing cutbacks) ... blank drives again (can't trust what came in as we didn't install it), install a fresh OS, reload from backup. (I left out the unrack / pack / unpack / re-rack, as you'd normally have to do that ... but that doesn't take much time, unless they send you back something diferent and the rails don't match).
The machine that had 2 RMAs I kept as a spare, rather than put it into service for anything that mattered ... it just wasn't worth dealing with the headaches from it ... not only was there the 2 months from RMAs, but procurement takes between 1-4 months, depending on if anyone bothers bidding when the SEWP request goes out.
Say what you will about Apple's OS ... the hardware's very reliable, and the minis are cheap enough that it can be put on a government purchase card when you need one without waiting 2 months. My only issue w/ running Mac Minis as servers is the single-tap power. Well, that and thunderbolt, but there's two thunderbolt taps on 'em now, so one for the storage, one for the KVM. (but I won't need the KVM if I'm not running MacOS).