The difference between full ROM and magisk ROM is, that the magisk ROM will only be applied as magisk module and your stock system will be untouched, so that you easily could install HTC ota updates, by just disabling the magisk ROM module.
(As long as you keep the stock recovery, else you have to switch from twrp to stock recovery, before you apply a HTC ota update)
If you flash the full ROM, your stock system will be replaced by the modified system of the viper ROM.
You there could select, which system apps you want to be removed on flash and several other stuff.
You also could select, during the aroma installer, your preferred root method.
Recommended is magisk+magiskSU.
Regarding tweaks, there isn't a real difference.
Just a very minor amount of tweaks doesn't work on the magisk ROM.
E.g. changing the boot- and down animation.
Sent from my HTC U11 using XDA Labs
Thank you for clarifying. I'm backing up a stock image and I'll be good to flash away again. It feels good to be rooted again