Had the standard models.
Have the 20th models.
Vastly different.
The XTC is a more versatile amp overall; as mentioned, I have the 20th EL34 model and that's because it can get to where the 101B goes but the 101B can't go where the 20th can.
The 20th Sheevs is amazing. Cleans are nutso good - round - potent - so full and powerful - and the fact that the boost keeps the tone-circuit engaged is super handy. The 20th Sheevs can get very aggressive - I've had some people question me on this but by using the MODE and SHIFT adjustments, this amp can get super tight, searing hot, and percussive with razor like gain, or notch it back a smidge and get super heavy grind.
The simplicity of the Sheevs can be an enduring quality to it as a stage amp over the hyper-complex (with respect) 101B or 20th XTC. The Sheevs is a beast - period. I am not typically a fan of 88s, but in this particular amp, they work incredibly well. It is super potent - very loud - but holds well cabinet dependent.
I can conjure all manner of tones from the 20th Sheevs - cleans, jazzy shit, blues, AIC/Jantrell, hot-rodded Marshall with more of that chewy Bogner goodness, nice low-end... Fucking choice amp.
Hope this helps.
Mo