Well put me down for someone who upgraded from an Inspire (X3) to the Phantom 4. I'll go over the key points of why I chose to sell my Inspire after a few flights with the Phantom 4.
Stability in wind - I've had my Inspire in some pretty strong wind to the point where it's on a really high angle just to stay in place... which is impressive to see and fly but unless you're taking photos or static video shots it's not useful for getting good shots. I haven't tried the Phantom 4 in strong wind and I don't expect it to perform as well as the Inspire but I imagine it would be close enough to be usable.
Setup Time - Not even close, 30 seconds from backpack compared to the 5 minutes if I rush on the Inspire
Hand Launch/Landing area - I haven't been comfortable hand launching the Phantom 4 yet as its idle motor speed it crazy high and I'm worried it will try and flip if I hand launch. I may or may not figure it out but having a much smaller landing area really helps where I hike/fly... either way I can easily hand catch the Phantom 4, I'm not going to hand catch an Inspire.
Camera quality - I did a side by side comparison and the Phantom 4 IS better than the Inspire X3. It's real close and not as big as the difference between the GoPro and Inspire X3 but the Phantom 4 definitely is better. Wether it's the 3D noise reduction or the sharper glass?
Weight - Once again not even close... I've hiked 27km with the Inspire + 3 batteries + Osmo handle. The Phantom 4 is a fraction of the weight.
Speed - On paper this seems pretty close or with the Inspire slightly faster... Having flown both I believe the Phantom 4 is much faster than the Inspire with Sport mode... it is crazy fast and crazy fun and since it still uses GPS it's fun to have it swoop up at the end and stop.
Build quality - The Phantom 4 is kinda slippery with its new finish but it feels solid and sturdy. My Inspire has always had a bit of wobble in one of the arms and I've had to blow the dust out of the "guts" of the Inspire a few times, the exposed wires also may rub against other parts and I always had to check to make sure they weren't wearing down. Even the styrofoam case with the Phantom 4 is better built than the Inspire case.
Props in shot - Yes the Phantom 4 occasionally gets the propellors in shot... at the same movements that my Inspire got the landing gear in shot. NOT AN ISSUE
Range - I assume they're similar or the same with distance (I rarely go beyond 1k) but the extra ~10 minutes flight time makes a difference.
Battery - Not even close... almost an extra 10 minutes from my TB48's
360 camera - Okay this one is closer than you guys think. I heavily used the Focus Mode on the App AutoPilot (I think it's great and DJI should have their own). If you've got a dedicated camera operator then the Inspire wins hands down but with Focus Mode you set the camera to point at a spot and no matter where you fly the camera is always locked on that spot. Well... I'm happy to say that ActiveTrack does this too. Maybe not as well but it's close enough. BTW I've seen a lot of guys testing ActiveTrack by using it as an advanced Follow Me. I did a comparison between Follow and ActiveTrack and once you know the limits of the tracking then it's great.
X5/R camera - Here in Australia the X5 camera costs more than what I paid for my Inspire (don't even want to look at the X5R). All the footage I've seen shows marginal improvements over the X3. For me it's the piloting/movements/light which have the biggest impact on footage. Since the Phantom 4 camera is between the X3 and X5 and still allows ND filters it's good.
Collision Avoidance - Where/how I fly I don't have a huge use for it but I'm sure it will come in handy one day.
As sad as I was to sell my Inspire I'm very happy with the Phantom 4 and glad I was able to sell my Inspire for twice the cost of the Phantom 4.
I agree with all the points above from James. Iv owned and loved my Inspire 1. However, I have fallen in love with the Phantom 4, The portability of having it up in the air in 30 seconds is so much easier. I always took care of my Inspire 1, so I never transported with the camera on the gimbal (I know people do this, but I don't). Also the room/weight of carrying around the Inspire and the case can be awkward. The new styrofoam case is really durable and much more smaller foot print. If you get a backpack its going to fit nicely and easily come in and out.
Also I am really impressed by the image quality, the Inspire 1 gets slightly bury around the edges in 4k (not as crisp as it is in the center). The phantom 4 appears really crisp all the way around the picture, like James said, I am not sure what it is, but there is an improvement.
The Battery life is really great with the Phantom 4! makes it easier to go exploring with out running out of battery, only thing now is I need a bigger SD card for all the extra footage hehehe
The other thing is the cost, the Inspire 1 is really expensive and while all care is taken (and I have taken out insurance) it is a lot of money in the air that could potentially have an issue and crash, being in Australia its going to be a lot harder to get fixed.
While I love the look of the Inspire 1 and think the technology is awesome, the more I fly the Phantom 4, the more I think to myself that i'm not going to be really using the Inspire 1 any more. So I will too probably be selling my Inspire 1 as well, it has been good, but the Phantom 4 fits my needs.
If your a professional camera operator and do it for your job or profession - I would say go for the Inspire. If your a hobbyist, enjoy recreational flying, or want to capture nice moments of yourself, family, friends, etc, then the Phantom4 is more suited to you. Remember the Phantom 4 is also 4K so you could get away with doing professional shots with it as well.