you can use any of them, provided the right settings. When a parallel loop is available though, it's recommended over serial ones for time domain FX.
When using a parralell loop, set/dry should be set on 100% wet as the dry is already provided by the internal circuit in the amp.
Well…. life is made of choices! Something gained and something lost!
The audio quality you'll get from a parallel loop is much higher though.
I would go for parallel without hesitating a second. You may want to experiment and decide what's your choice. Isn't that nice?