You may be considering buying your first camera or just planning to change or upgrade your existing camera system. You may want to consider mirrorless cameras besides the DSLRS which all Professionals and amateur photographers use and trust today. While selecting, some of the key features to look for in a Mirrorless camera include the following

Full Frame - As with a DSLR, a full frame sensor will give you much better performance in all respects as compared to a crop sensor. Especially High dynamic range and great performance at higher ISOs

4K - This is the resolution for video that is soon going to be the standard, if not already and many fields. Do remember that the file sizes of raw 4k videos as you shoot are huge and you require a bigger and faster memory card as well. 

Viewfinder -Mirrorless cameras do not have optical viewfinders. If the viewfinder is absent, you will not be able to shoot with the camera closer to your eye as you do with DSLRs. Having a viewfinder - electronic or rangefinder type is certainly a plus point.

Tiltable screen - Its good to have a tiltable screen as that enables you to shoot from many possible angles otherwise not very convenient to shoot. Vertically rotating (upward-downward) screen is better than the one which rotates sideways. 

Tilt able screen lets one shoot conveniently even from odd angles. Also lets one shoot at waist level angle.

Megapixel - More is always better.

Weight and compactness - You are buying a mirrorless camera because you do not want to move around with a heavy camera. Go for lighter weight camera otherwise the very objective of buying a mirrorless is defeated. 

There are very limited options today (september 2017) in terms of MIrror less cameras which have full frame sensor along with 4K shooting capability. 

Sony Alpha a7R II

• Full Frame Sensor: 42MP 36x24mm Exmor R BSI CMOS
• Video Capture: UHD 4K at up to 30fps
• Image Stabilization: 5-Axis SteadyShot INSIDE
• Continuous Shooting: Up to 5fps at 42MP
• ISO Sensitivity: 100 to 102,400
• Shutter Speed: 1/8000 to 30 seconds
• Autofocus Points: 399 Phase-Detection / 25 Contrast Detection
• Viewfinder: 0.5-inch 2.36M-Dot XGA OLED Tru-Finder (Electronic)
• Display: 3.0-inch 1,228.8k-Dot Tilting LCD Screen
• Built-In Connectivity: WiFi, NFC
• INR 219990 Body only
• sony website
• Download Brochure

Sony Alpha a9

• Full Frame Sensor: 24.2MP 35.6×23.8mm Stacked CMOS
• Video Capture: UHD 4K at up to 30fps
• Image Stabilization: 5-Axis SteadyShot INSIDE
• Continuous Shooting: Up to 20fps at 24.2MP
• ISO Sensitivity: 100 to 204,800
• Shutter Speed: 1/32000 to 30 seconds
• Autofocus Points: 693 Phase Detection / 25 Contrast Detection
• Viewfinder: 0.5-inch Blackout-Free Quad-VGA 3.7m-Dot OLED (Electronic)
• Display: 3-inch 1.44m-Dot Tilting LCD Touchscreen
• Built-In Connectivity: WiFi, Bluetooth
• INR 329990 body only
• On sony website
• Download Brochure

Leica SL (Typ 601)

• 24MP Full-Frame CMOS Sensor
• Leica Maestro II Image Processor
• Internal DCI 4K Video & L-Log Gamma
• 0.66" 4.4MP 0.8x Electronic Viewfinder
• 2.95" 1.04M-Dot LED-Backlit Touchscreen
• Contrast AF System with 49 Fields
• Up to 11 fps Shooting and ISO 50000
• Two SD Card Slots; 1.3" Top LCD Screen
• Weather-Resistant Milled Aluminum Body
• Built-In Wi-Fi Connectivity and GPS
• Leica India website

Between the two sony cameras Alpha a9 overall has better features if you are into sports, street, wildlife or any other genre which requires a fast and continuos shooting along with more number of focusing points. Higher shutter speed of 1/32000 is great for outdoors. If higher resolution is the priority then 42mega pixel will certainly give you far more detail than 24.2 mega pixel.

There are other mirror less cameras with 4K but they don't have a full frame sensor. 

In my opinion, go for Sony a9 if you are on the go and need a fast camera with the ease of faster focusing. If you shoot in the studio and under controlled conditions having a better resolution might actually be your priority. Go for a7R II, which will in no way disappoint you outdoors in demanding situations.

I am currently shooting on Phase one - Mamiya - canon systems.  

Both Nikon and Canon DSLR cameras are also good but when it comes to mirrorless cameras I think Sony is the best available option with both 4K and full frame capabilities. In days to come, if I add a mirrorless camera, it probably would be either sony a9 or Sony Alpha a7 R II

The prices may not drop but technology certainly will get more perfect with passage of time.

Munish Khanna is a Delhi, India based commercial photographer who has successfully trained countless photographer across the globe over the last two decades. Munish Khanna is considered as one of the top ten advertising and fashion Photographers of India.