I have a Digitech JamMan Stereo. http://digitech.com/en/products/jamman-stereohttps://www.thomann.de/gb/digitech_jamman_stereo.htm
A brief history of how I came to get this one:
I started playing with the looper function on the Zoom G3X, but this has some very very serious limitations, not least the recording time limit, and it didn't seem possible to have more than one tone. So then I bought one of the NUX Loop Core as above. This is allegedly a copy of the Boss RC-3 but at a lower price. It worked perfectly well, but with the proviso that it has only one button - so if you want to use it on the fly or live you need to buy a complementary footswitch. I still might buy another one to use with the bass rig, so as you can tell I was perfectly happy with how it sounded even if the operational issues made my head hurt a little - just getting used to it would be the trick there.
Which brings me to the JamMan Stereo. Loads of buttons
so you can record, overdub, stop and start a loop easily. Unfortunately you have to bend down to press the 'store' button before you can move on to the next loop. The two top switches move through the loop register and you can set it to change the moment a switch is tapped or to wait until the current loop phrase reaches the end. You can swap and copy loops obviously, but it is no Chilli Monster.
It's not perfect, but it does vocals as well as stereo, has 198 loop memories and takes a 32gb SD card so effectively could hold several gigs worth of backing tracks (up to 16 hours). A few other features from the first page of the manual:
aux input to import backing or prerecorded tracks
3 stop modes, instant, finish, and fade
It has only a basic rhythm function and most importantly the JamMan software is incompatible with Windows 10 and Digitech show no signs of wanting to deal with that.
As with all loopers getting the start an stop in the right place can be a bit of a tricky thing to get right - I still get it wrong at least half of the time, but it's great fun.
Using it in the effects loop of the Katana, as long as the effects loop is set to after the on board processing means that I can record a loop using one sound and then switch to another to overdub or play along. Hours of fun (unless you are forced to listen). I haven't tried to use it with the Mustang 3, but I have it in the back of my mind that you can change the signal path on that to get the same result.