In terms of performance, the RG503 lies between the RG552 and the RG351MP.
It also (currently) only offers a Linux OS, not Android like the RG552.
All systems up to and including PlayStation 1 run perfectly and look great on the OLED display - however, it still struggles with N64, PSP and DreamCast (but this could improve with software updates, as was the case with the RG351MP).
You can also connect it directly to the TV via the HDMI port and play via a Bluetooth controller.
What makes the RG503 special, however, is the pre-installed Moonlight client, with which you can stream PC games latency-free to the small device in your home 5GHz WLAN network (assuming a PC with a reasonably up-to-date NVIDIA graphics card).
This means you can comfortably continue to play your PC games in bed.
The battery life of the RG503 is about 5 hours, depending on the game.
Who is the handheld best suited for?
For anyone who likes to play retro games on the go or on the TV, but who would also like to stream a current game from their PC onto it.
The handheld is charged via USB-C. A charger is not included, but you can charge it with any standard USB power supply or a PC.