Buy whichever one makes you happy. When the KX3 first came out I drank the Kool-aid and thought it would be the perfect radio to fill my portable and compact base station needs. I sold a bunch of stuff to fund a fully loaded KX3, and owned it for about a year. For as much money as I had wrapped up in it I didn't use 50% of the available functions. I just wanted a radio to turn on and run, and not screw around with settings. I used it portable, but I was too scared of the long knobs and big shiny unsealed faceplate to take it on any hardcore adventures. To make up for it's fragile feel I put it in a Pelican case which negated any weight and size advantage it had over other radios.The filters are great for CW use, but for SSB the Noise Reduction and Noise Blanker features did very little for me, and the echo effect of the NR was just as bad as whatever noise I was trying to reduce. I thought it would be great for digital modes as you didn't need any fancy interface, but on slow modes like WSPR and JT65 it drifted way more than was acceptable for my uses. The 2M module was weak sauce and expensive so I never bought it. The lack of 70cm coverage was obvious.
I've since bought a fully loaded FT-817ND with DSP, speech processor, and dual filters and it meets 95% of my needs. The biggest things I miss about the KX3 is the stereo effect audio on SSB, and the built in tuner. IMHO the FT-817 BHI DSP works much better than the NR feature on the KX3. I like the rugged feel of the FT-817 and have even used it in mist & light rain and don't feel like I'm going to hurt it. I backpack with the radio and like being able to leave it in the radio pouch and still access the faceplate without removing the radio. My biggest dislike of the FT-817 is the 400ma standby power draw, but I compensated for that by running it with lightweight 5ah LiPo RC batteries. I picked up an Elecraft T1 tuner and while it's not as streamlined as a built in tuner, it gets the job done. On digital modes the FT-817 is rock steady, even during long, full power TX periods.
I don't regret buying the KX3 as I learned a lot from it, and it helped me nail down the features that I really wanted and needed in a radio.