To a great extent it depends where I'm going to be riding, the weather conditions and how long of a ride I expect.
I once made a weekend trip during the summer heat, wearing my armored motorcycle jacket, and carried a holstered full-sized pistol (SW99 .40 S&W). I carried a couple of spare magazines on my person and several spare magazines in a case packed away in my backpack. The altitude changes made for cooler weather in the mountains, part of the time, but it was miserable in the flatlands. Ugh.
Next similar trip I made I decided to take my 642-1. I carried it in a small digital camera case, worn on my belt, which also holds 5-6 speedloaders, but doesn't even look big enough to hold the gun. (The kind of case which does NOT use velcro on the belt loop, BTW.
) Even carried a couple of speedstrips in my pockets and a spare box of ammunition in my backpack. I had more ammunition than I had carried when I previously carried the larger pistol and spare magazines, but I hardly knew I had the gun with me. I was able to ride as the weather required or permitted. Very comfortable trip. Of course, the armored jacket stayed home and I took my regular leather riding jacket, as well as a lighter weight jacket packed away that I could switch to wearing during the ride.
For just riding out & about in the Santa Cruz mountains or over on the coast I generally opt for the 642-1, either in a belt-case or inside a jacket pocket. The belt-case permits some shifting of the weight and placement on my belt. Lets me ride wearing a T-shirt, too. (I know about sleeves and such, so save your breath.
) Kind of like a little 'gun fanny pack', but much less obvious. It's fooled more than a few of my peers when they discovered I was carrying an off-duty weapon, and not a camera or PDA.
Depends on how I feel, and what the weather's like. I ride for enjoyment, and enjoy some comfort while riding, too.
If the weather's cool enough that I know I'll be able to comfortably wear a jacket even while off the bike, or if I'm wearing a loose, long-sleeved shirt as a light riding liner under the jacket (and can remove my jacket while off the bike), I'll sometimes choose to wear a compact/subcompact belt-holstered weapon.
Just depends. How comfortable do I want to be ... considered against what risks I perceive, or feel I should reasonably anticipate, depending on my planned riding for the day.