You need to figure out what compromises you’re willing to make 
Pros of Inpulse 500:
- unbalanced 1/4" outputs
- AUX IN (RCA and 3.5mm)
- balanced MIC IN
- feet to raise it up
- slightly sturdier build
- USB-B connection
Pros of FLX4:
- djay iOS friendly (convenient backup and charges iOS device)
- USB-C connection
- similar button layout to Pioneer club gear
FWIW - I use the FLX4 because I like being able to use both djay macOS and iOS, but, added an external mixer to get the better audio jacks and AUX IN. Plus, the FLX4 layout makes it easier for gigs with in-house Pioneer gear.
