I had a 20th EL34 XTC last year for a while and currently have a 101b (traded a 20th Shiva for it and will be trading back this week).
My band does mostly 80s rock stuff so a hot rodded Marshall tone is what I am usually after, but I have always been a fan of Cantrell's tone and got into Bogner more or less looking for that kind of signature grind. I wish I was able to A/B the 20th XTC I had with the 101b, but here are my thoughts from what I can recall.
-The Excursion settings seem pretty different. On the 101b I can set it to Tight and push the Bass a bit more and its big and full, but a little more compressed and as the name implies...tighter. On the 20th I always felt like when I went to the Tight setting the amp went too thin overall. I had to use M or L on that amp.
-The Blue on the 101b seems to have a little more gain. It is a bit smoother overall and more compressed so it could just be some of those subtle differences making it seem like it has more gain. The 20th was more open and a little more Marshallish in a way. Sometimes I liked that, but I wanted more gain and at the end of the day it didn't seem to be the same coveted Blue channel everyone loves on the 101b.
-The Red on my 101b has been modified to the Cameron specs. It can be set to sound big and dark and smooth, or brighter and more edgy. That is where I would play metal, but its not instant metal inspiring tone. Plenty of gain here though. In fact with it on R1 and M or L on the Excursion and it goes microphonic gain bananas pretty quickly.
-Green, well I don't play cleans a whole lot. I don't know that the EL34 version of the 20th really does those changes any justice. I think the clean on the 101b sounds pretty damn good. If I recall correctly I tended to use the boost on the 20th to get the clean sounding nice and big and full while doing the peg the volume and use the Gain for overall volume trick. On the 101b the boost on the clean doesn't sound good to me. Maybe for single coils, but with hot humbuckers its too ratty of a breakup.
-The 20th did sound boxy and congested a few times when I wasn't able to open it up. On the other hand, one outdoor show I did last summer on a nice big stage, I had that thing cranking and it was a ton of fun. Sounded great and was really easy to connect with the feel.
I am not a metal player...the most metal I get is some Metallica and the occasional Slayer lick. I am more hard rock, but like aggressive tones and tighter amps most of the time.
I traded my 20th XTC last year for a Helios. I liked the amp, but didn't feel like it was a great fit in the band along side the Splawn Quickrod my other guitar player uses. I was happier using it with the other band I was in at the time where I was doing a ton of different kind of music and was the only guitar.
I pounced on the 20th Shiva recently because while I love the Helios it is a one trick pony in a live situation. Great hot rod Plexi and covers me most of the night, but left me wanting when I needed a heavier tone. I think the Shiva is meaner sounding than the XTC, but they have a different voicing and feel for sure.
I feel like I am all over the place in this post. Sorry if I made it more confusing. I shoulda just said amps are cool, Bogners rule.
Good luck...look forward to hearing your take on whatever you get in the future here.