Well, for the sake of discussion, I'll offer another point of view.
I'm 67 years old, I've been using my 9" SB, the very same exact lathe, since I was 8 with my Dad's supervision, and since I was 10 on my own (that's 59 years more or less as a hobby user) - so it isn't like I just fell off the turnip truck when it comes to using a lathe. I like my 9" SB, no question about it. I used it again yesterday and enjoyed it - it's an old friend that I'm comfortable with and within it's size limits, it does good accurate work, the ways are essentially perfect, and it won't be for sale any time soon.
But, with all due respect for the heavy 10, which is basically a 9" on steroids, if I had to pick between a new in the box 10H and a Harrison M300, or similar lathe, or a well made 12x36 import (gear head or belt driven - not from Harbor Freight), for doing gunsmith work, I'd pick the Harrsion or equivelant or well made 12x36 import) in a heartbeat and never look back.
If I was going to make a living as a gunsmith, I'd go for the Harrison or the bigger Grizzly. I would definitely pick a lathe with a 3ph motor. A lathe with a 2 speed 3ph motor would be even better since all the spindle speeds would be available with out a belt change. (Changing the belts on the SB is easy, changing the one on the Andes is more difficult.) The ratios on the Andes are such that I could replace the existing 3ph motor with a 2 speed 3ph motor, rewire it to add a Hi-off-Lo speed switch, and never have to change the belt, but I've not done that.
I couldn't find a 10H when I was looking for a new lathe in the early 1980's ('81 or '82 I think) so I swam upstream against the conventional wisdom on RCM and bought the Andes, a 12x36 Taiwanese gearhead. I picked it instead of the Grizzly because I started with a lathe that had a slow speed around 36 rpm, the slow speed on the Andes is 36 rpm, and I didn't want a 70 rpm minimum speed. I've never regretted the choice. I would look at bigger lathes now and then (always imports), but just couldn't justify the purchase as a hobbiest. Besides, with one exception, the Andes has been able to do everything I've asked of it. (It couldn't turn a new axle bearing hub on the end of my '31 A Roadster rear axle housing - the shock mount needed more swing than it had and I think the bed was a bit too short).
Since I retired and became interested in gunsmithing as a fun way to spend winters when it was too cold to to hunting or to the range, and a way to have better rifles than I could afford to buy, I've discovered what a nice lathe the Andes is for doing gunsmith related projects. Fitting and chambering barrels is easily accomplished through the headstock with the addition of a few easy to make tools and a minor spindle modification. Truing receivers is not a problem - the ways are hardened and ground and true within a tenth or two over distances of interest so making mandrels, and other tooling isn't a problem.
There is nothing wrong with a 10H, it was and is a good lathe, but in my opinion, while it is still an option, there are better lathes available for gunsmithing now.
If I could only have one lathe, I'd keep the Andes because it is so much more capabile than the 9" SB.
You pick your pony and take your ride.
Fitch