Controversial question - but not as bad as the oil debate though
Every rider will have an opinion.
If you are competing, then a rivet link is more secure but, for everyday use, the split link is much more convenient in my opinion.
In addition to the points already made, don't forget to change the split link when the spring clip gets worn. I reckon two front sprockets and split links to one rear sprocket and chain
I confess to having bought a couple of riveting tools over the years and never managed to get one to work properly. They either bent or broke the pins on the tool if the heads hadn't been ground off the rivet link I was trying to remove. Defeats the object if you need to remove/replace a link out on the lanes
Personal opinion but I have rarely used soft links on anything other than 100 plus bhp. I carry a spare split link on the bike so order two. Don't forget fishy swimmy uphill
I've only ever used split links, much easier and still very reliable. Oh once about twenty years ago the clip popped off after rolling the bike backwards. But the chain guide was bent. Wasn't on a TTR either, a KTM by memory. So I always carry a spare old link with my tools.
The new split link will come with O rings.