I came from the Z3v (Verizon Wireless version of the Z3) to my HTC Desire Eye because I changed carriers (Z3v can't do LTE on AT&T). Do I miss it? Yes and No. I picked up the HTC Desire Eye over the M8 mainly because of its waterproofing and size (similar to Z3v). At the same time I got the HTC, I decided to try a Z3 Compact(very similar to a Z3) as well to do a direct comparison.
Overall, the Z3 line is a great phone! Battery life is amazing, Sonys overlay is minimal, and hardware is nice... but the camera software needs some work. It has lots of options, but slow shutter speed made me CONSTANTLY miss shots of my cats, cars, etc.. I didn't realize how much I missed a fast(er) camera until I picked up the HTC Desire Eye. Also, it turned out that what I was missing most about the Z3v, the Z3 and Z3 Compact dont have - Wireless Charging (neither does the HTC for that matter, but Im making the point that my reasoning for desiring Sony was weighted towards something not available on many of their phones). And with the waterproof flaps on the Z3, I am just tired of opening flaps when i want to charge.
So, primarily due to the camera, I have returned the Z3 compact and stayed with the HTC. The Z3 doesn't offer anything (except a bigger battery) over my HTC Desire Eye and that is not worth having a camera that (while you can get good shots) is not able to catch those "moments" when you need a camera.
If Sony fixes the camera would I go back to Sony? Absolutely. Especially with the Z4 on the horizon (no flaps to cover charging ports!). The battery life and hardware of Sony is so appealing to me. But for now, I will rock the HTC and will enjoy taking my pictures!
Since you have a M8 instead of the Desire Eye, the waterproofing might be another bonus for upgrading. But I HIGHLY recommend checking out the camera beforehand.