I've used both & both are solutions I would use - I had issues with G-Data's firewall with a music config I no longer use the tech @ G-Data seemingly don't know how there own firewall runs - Otherwise try them both - If I had to choose I would use Kaspesky IS as I am now
I haven't tested GDATA's firewall but I'm not very impressed by ESET's Internet Security either. IMO the feature where it scans your whole subnet for devices and alerts you about changes is odd to turn on by default. It's pretty ARP spammy which impacts wifi performance especially.
I had ESET block both Sonos speaker discovery and also wireless display mirroring silently, and the automatic rule creation wizard made incorrect rules for both of them (too specific, won't work when IP addresses of the destination changes). So even though my license for ESET is Internet Security, I always downgrade to NOD32 for my machines.
I know this isn't something you were considering, but honestly for me Norton's firewall and IPS has been my gold standard for the best combination of protection and not interfering with legitimate usage. I'm also interested in what other suites offer similar protection but most firewalls have caused me more grief than is worth the effort of keeping them configured.
Pro active detection and prevention : G Data.
Static signatures : Eset
Unknown malware cleanup and repair capability: G data
Behavior based detection against zero hour malware / ransomware : G data
Interface : Both good, may be eset hikes
Impact: Both are comparably light.
Offline protection with Pro active based detection (Not dependent on cloud) - G Data
Web level protection : Equal.