It probably won’t be long; someone already got code for VIA-compatible GMMK Pro firmware merged into QMK. So technically you can use it with VIA now if you compile that and flash it to your keyboard.
As for the practical differences, with QMK if you don’t want to get into code you have to use QMK Configurator to configure your keymap. When you’re done choosing your layout you hit “compile”, download the hex file, and use QMK Toolbox (linked on the webpage) to flash the hex file to your keyboard. You’ll also want to download the “keymap.json” file which you can re-upload to the site to continue editing the layout from where you left off. Any time you want to change the keymap you have to go back to that web app, upload the json, make your changes, recompile, download the new hex and json, and go into QMK toolbox to flash the new keymap to the keyboard.
With VIA, you use QMK Toolbox to flash the firmware one time then you use the VIA desktop app to make changes to the keymap. Any changes you make in the software instantly update what the keyboard does; no compiling, no flashing, no trying to figure out where you saved that old json file.
Frankly, VIA is a better solution for people who don’t care about learning about how firmware works and just want to customize their keyboard layout. But if you don’t make changes to your layout much then QMK is easy enough to work with.