That's quite a different pair of amps.
The Katana is something of a one-stop amp. A wide variety of tones, built-in effects, loud enough to gig, I think the head even has a little speaker built in for practice. More tweeking available if you connect a computer. And relatively light, as all SS amps are.
The Marshall is a bit of a one trick pony, compared to this. But what a trick. With the Marshall, you'll probably want to add an overdrive pedal(s) to get the range of tones you want - and some mod pedals down the road. So the Marshall is like one part of a system you'll have to assemble yourself. But it will be your system, not the same system as all other Katana owner's have.
So I have only two questions for you. Red pill or blue pill? And how deep down the rabbit hole do you want to go?
I’ve tried the Marshall Origin 50H three times, and the BOSS Katana Head 2 times, using a 4x12 for both amps. Every time I used the Marshall Origin, I placed a pedal in front of it. The first time, I used the MXR MC402, second time, Ibanez Mini Tube Screamer, 3rd time, BOSS BD-2. I got good results out of all of them. When I tried the BOSS Katana Head, I just used the amp. I plan on setting one medium gain tone, which I’ll include an overdrive pedal to boost if needed, rolling down my guitar volume to clean up, sometimes turning on a mod effect in the FX loop when I clean up, and using the BOSS GE-7 I have through the FX loop to turn on for a lead tone, then bypass it to go back to my rhythm tone.
I’ve also lifted both of them before. I feel that I can lift both of them conveniently.
Ps. I have a drive pedal already.
I play lead guitar at church, but I plan on starting a rock band. The frontman of the church band plays rhythm guitar, and when playing with him, I feel like I’m playing unmixed with a drummer while using a Line 6 Spider V practice combo.
I currently use a Fender Mustang V2 15W, which I feel has insufficient built in distortion. It also offers no control over midrange or presence, which are both crucial to the sound I like to use. I also like the sound and response of a tube amp, and the lack of speaker-induced weight in a head. If I were going for a big amp like the two I’m trying to choose from, I’d probably want to use a head, since it’ll be lighter than a combo.
Ps. I don’t really plan on making much use of the Katana Head’s built in speaker.
About Mod Pedals, I would like to have the BOSS DC-2w, which I still have yet to try. Your average chorus effect is quite wobbly, while I’m looking for the more crystal clear sound of the Roland Dimension D, which was adopted by the BOSS DC-2, and recreated in the second mode on the BOSS DC-2w.