If your main concern is defense against humans, I would go with the 1911. I like .357 revolvers a lot, but a semi auto is going to be a lot easier to reload and with both of these being low round count weapons, I think that’s important.
Yes there are speed loaders available for revolvers, but when was the last time you practiced (quickly) unloading your revolver and then reloading it (quickly) with a speed loader? Most people don’t - it’s a different process than a normal plinking reload. But loading a semi-auto quickly is going to be much more similar to how you load it when plinking.
I know this is a gun forum but I would focus more on the primary activity (which is being on the bike) and how that affects your carrying of the gun, rather than which gun is better for shooting.
What kind of riding are you doing? “Adventure riding” can be a lot of things, so what percent of road to off road, and what kind of off road (well maintained dirt road, unmaintained double/single track, or even trials type terrain)? That all gets to the type, severity, and frequency of potential crashes. In short, a crash on the road is more likely to be at higher speed and involve some impact while you’re vertical on the bike, while off road probably has a higher chance of being at a lower speed and losing traction (so single bike just falling down). At the extreme end (trials type riding) you can sometimes just step off the bike when it goes. (For those unfimiliar, trials has a lot of small/light bikes and emphasizes slow speed balance rather than getting through a course quickly - this is where you get a guy balancing a stationary motorcycle on a log for 2 minutes without putting a foot down)
Anyway, it is much more likely that you’ll crash while wearing the gun than actually have to use the gun, so I’d make sure to cover that base. If you have the gun around your waist, you have a concern about breaking your pelvis or hip, and if it’s close to 5-6 o’clock, your spine becomes a concern as well. A shoulder holster puts it next to your ribs which can lead to bruised/broken ribs and worst case punctured lungs. From what I have seen in motorcycle accidents, I would probably lean towards a shoulder holster as that seems to be a less common area to injure compared to pelvis/hips.
I would also recommend finding a way to get a separate piece of D30 armor between you and the gun (and mag/speed loader carrier). I would try to attach it to the holster itself rather than rely on a separate piece of armor.
And finally, make sure you can use the gun while fully geared up. That includes practicing drawing and changing mags. Make sure you can do everything with the gloves you wear while riding. Depending on your gloves, sometimes the armor can be a little weird and you don’t want to find those quirks during a gunfight.