I’ve only ridden a Stache, and its a solid bike. I’m sure Farley is solid too. I see it more as fatty vs. plus size, so you need to think about your riding style and where/when you ride. For me, I ride some fatty in the winter, but its nowhere near as fun as the 2.2″ tire the rest of the year – the speed and handling is why I ride. But I know some guys ride fatty year round and love it.
Test ride them both. Fatty will outperform in the winter, but think about (and test ride) the rolling resistance in the summer. For me, if I had to choose just one, I would go with a plus size as the year-round choice.
As @TK34 says, it really depends on what you intend to use the bike for as well as your riding style. There’s a world of difference between a CX bike and a fat bike. It’s basically the two extreme ends of the spectrum. While there are others that ride their fatbikes year round, I find that there’s way too much rolling resistance for my style of riding. If you want to shred singletrack – climb and descend – you should strongly consider a trail bike. Again, depending on your needs/trails, you can choose a hardtail or FS. With a trail bike you get a lot more versatility overall and that can be further extended based on the amount of travel you choose. If you buy a bike that is compatible with both 29 and 27.5+ wheels it’s almost like having two bikes (although you’ll need to invest in the 2nd wheelset). However, I personally would not let that last point drive the decision.
The ideal Fatbike has 26×4.8in tires on e80mm (e=exterior width) rims and has no suspension. This bike will perform very well in snow but makes a crappy trailbike. The Farley comes with 27.5 Fat wheels. There are vey few 27.5 Fat tires made. So if you want a Snowbike, look for a different brand.
A 29+ hardtail makes an excellent Trailbike. However, a Plusbike is a crappy Snowbike. 3.0 tires just aren’t wide enough to perform well in snow. The 29+ Stache currently comes with i45mm (i=inner width) rims and 29×3.0 tires. I keep hoping that Trek will update this bike with 29×2.8 tires on i35 rims. A 2.8 tire-i35 rim does everything wider rims and tires do but with less weight and rolling resistance. An i35 rims also gives you more versatility and allows you to use 2.4 to 3.0 tires. While an i45 rim is only good for 2.8 and 3.0 tires.
There is really no way for you to get a bike that does both well!!!!!!
It it was me, I’d say get both. However, I’d get a 29+ full-suspension Trek Full Stache (which has i35 rims) and a Borealis Fatbike.