I went to the local HRO store this morning and spent 1-1/2 hours with the FTdx-101D and 30 minutes with the 7610.
I went in really, really wanting to like the 101, because IMO it's the best looking and the best built of the two. Unfortunately, I came away disliking the 101, for reasons listed below.
My dislike of the 101 entirely boils down to its display. The spectrum scope is best described as "chunky", e.g. it looks very blocky and pixilated and not smooth. It's not an issue with the physical screen, because both the Yaesu and the Icom have 7" 800x480 displays, and the Icom spectrum scope appears smooth and not blocky.
In addition to looking blocky, tuning on the Yaesu screen was weird. I dislike "Center" mode and strongly prefer "Fix" mode. When tuning with the main VFO knob in Fix mode, the rig paints a red stripe on the screen who's width presumably indicates the width of the filter, and this stripe moves around when tuning, which is very distracting. Tune back and forth a few times and you have a sinuous s-curve of red on the waterfall. Why this red stripe doesn't stay vertical and move when tuning, I don't know, but this is a misfeature, and an annoying one. Compared to the Icom, the Yaesu waterfall seemed to have a lower dynamic range, even after adjusting the levels, etc.
I gave the 3DSS a fair try, but after about ten minutes concluded that it is a gimmick just like I thought it would be. Sure, it might help sell rigs (because no one else has it), but I can't see it providing any additional information in a more useful way than a 2D waterfall. Keysight doesn't have a 3D display on their megabuck spectrum analyzers, so perhaps they're not that useful after all.
The Icom, in contrast to the Yaesu, had a much better implemented spectrum/waterfall display. It was smoother, less chunky, and has several averaging options to smooth out the jumpiness. Since jumpiness adds no useful information to a spectrum display, the averaging helps a lot to make it easier to look at for long periods. Tuning with the VFO knob on the Icom was smooth and without weird artifacts like the red stripe on the Yaesu display.
As far as build quality goes, it seems that the Yaesu is better built, with better knobs and overall fit and finish. I would definitely prefer this over the Icom, but the Yaesu's poorly implemented display ruins the experience for me. I also like the fact that the Icom uses RF direct sampling as opposed to the IF sampling Yaesu uses. This is the future, and will become more prevalent and better as ADCs and software become faster and more accurate. Icom will have a one generation lead on Yaesu that will probably become clear in the next generation of rigs from both companies. The Icom also has an I/Q output via USB that the Yaesu lacks, and an Ethernet port rather than the obsolete RS-232 port on the Yaesu--how many modern PCs have an RS-232 port? Very few. Probably 99+% of users are going to use the RS-232 port with an RS-232 to USB adapter, so why not just put a second USB port on the back? The Yaesu has 200 watts of output, and although this might be nice, I do have a legal limit amp, so it's just a minor thing.
To me, the "killer" feature of recent rigs has been the waterfall display. I find this so useful that I could never go back to a rig without one. That's why my FTdx-5000 has been sitting in my closet for the last 6-1/2 years. Rigs like the Icom and Yaesu have displays that are useful, but at only 7" and 800x480 resolution, they're far from what they could be. Even my four year old iPhone 7 Plus has a display with 1920x1080 resolution, so it's obvious that the technology is there--Icom and Yaesu cheaped out and spec'ed a low-res display. The fact that both support external displays isn't a factor here, because both limit the external display to showing an exact copy of the built-in screen at either 800x480 or 800x600 resolution. That's a complete waste on my 32" 4K displays.
Since I spend most of my time SWLing the shortwave and BCB bands, I've had lots of recent experience with SDR receivers. All of my SDRs are simple boxes with antenna connector(s) and a USB port. The PC handles the display work, and all of the software that I've used (SDR Console, FDM-SW2, SDRuno, and HDSDR) support hi-res PC displays. It's really nice to see a big, smooth waterfall of an entire broadcast band on a 4K monitor at its native resolution. All of these software packages support extensive averaging functions to help tame the chaotic nature of an HF band and make tuning a pleasure.
With that said, I'll get into what's probably going to be a controversial topic with some people, and that's the option of going with a newer Flex rig. My primary rig for the past 6-1/2 years has been a Flex-6500, and that rig, both hardware and software, has been flawless in my shack. I've never had the hardware hang, freeze, or spontaneously reboot, nor has the software (SmartSDR) ever froze or aborted. And during those 6-1/2 years I've applied Windows updates too numerous to count, and not one has ever affected the Flex software--it just keeps chugging along. The best part of the Flex software is its panafall display, which appears on my 32" 4K monitor at full native resolution, and the waterfall scrolls perfectly smoothly--no stuttering or pausing, even when I'm doing other things on the computer at the same time (the computer is a seven year old Intel Core i5 with 16GB, so it's certainly no speed demon). It can do that because all of the real work is done on the CPU and FPGA in the Flex box--the only thing the PC does is handle the display and the UI. It's not like the Flex-5000A, where, for instance, the transmitted RF signal was generated in software on the PC and sent over a Firewire connection to the Flex box. I operate a lot of CW and cannot detect any latency when sending with a paddle on my Flex-6500.
So, my choice comes down to the Icom 7610 or the Flex-6600. If I went with Flex, I wouldn't get the 6600M model, because I'm not very happy with how they've implemented the front panel display (it's a Dell tablet sandwiched behind the front panel and all of the normal Windows stuff hidden from the user). Do I want buttons and knobs, or do I really care about that? If I get the 7610, it'll probably be mostly to have something different than my current rig. So why the Flex-6600 if I already have a working 6500? The 6600 has an additional spectrum capture unit (ADC) that permits things like diversity reception and better front-end filtering than the 6500.
So, a lot to think about. Flex has already announced their Black Friday deals on their stuff, and it remains to be seen what, if any, additional discount HRO/DXE/Gigaparts will offer on the 7610.