Nobody has brought up that dreaded disease older guys get, Arthritis. I would think a side lever would allow you to use a gun for for longer than a top lever.
I had a discussion with Toby Barclay at the Vintagers in Rhode Island one year and he said left side levers worked better for right handed guys.
I have never used a side lever gun and have never bought one. They are relatively rare to find in good condition, especially left sided ones.
Builder;
I (at 77 years of age) have the dreaded arthritis is both of my thumbs, and I can say with evidence that for a right handed person with arthritis a left handed side lever is not recommended.
I have a Stephen Grant right hand side lever, and yes it is easier to open than my top levers, considering arthritis. The problem with a left handed side lever for a right handed person is that when opening the gun with the LH sidelever the right thumb needs to move completely over the top of the stock grip and action and then press the side lever down enough to unlock the barrels from the action. My arthritis is in the joint of the thumb/hand portion--as is most often the case, and as a result the RH side lever works best as the RH thumb does not have to work alone to move the RH side lever as the entire top portion of the right hand can rotate slightly counter clockwise and cause the downward (while some of the fingers of the right hold the bottom of the action of the gun)movement of the side lever without stress the arthritic thumb/hand joint of the right hand.
I know Toby and for a youngster like him a left handed side lever likely works for his right hand. But I did not like a left hand side lever even before I came down with arthritis as the position of the left handed side lever interfered with my right thumb position during shooting sometimes. That issue and other issues were probably outlined by Stephen Grant himself to his customers when he took orders to build a side lever gun for a right handed person.
Now, I must say this that I rebuilt my 129 year old Stephen Grant side lever lock/unlocking mechanism including building a new action locking bolt, in order to remove excessive play in the side lever mechanism where it opens quickly upon depression of the side lever--as it would have worked on 10 Aug 1889 when it and its twin were handed over to the owner for the grouse shooting season on the 12th, as a matched pair.
