I have a Slide and a Leash
The leash is great for carrying smaller, lightweight cameras. I've happily spent a day carrying M4/3 sized and bigger on it with no issues whatsoever e.g. an X-T2 and the 18-55 which I suspect is bigger/heavier. It's much easier to swing into position (which is also a downside) without having to "flip" from grippy side to slippy side which IMO is better for things like street photography or travel.
Rich's downsides are the only 2 really, but it will depend on your photography.
If you're carrying a heavy lens, then it will likely cut in. I wouldn't carry my X-T2 and the 16-55 for example, or a 70-200 for long periods of time. The Slide comes into its own with heavier equipment as the weight is distributed far better and the emotional security of that fat strap is nice (even though it's the same dongle things that actually attach!). The grippy pads stop it swinging about (especially when you bend down) removing the risk of the camera swinging around.
The fact that the leash has no "grippy pads" means it slides about far more than the Slide! So if you have a long lens it can move about a bit. Also, when you crouch or bend down, the camera can swing forward. Manageable if a) you don't bend down much and b) you carry smaller lenses as less chance of it bashing in to stuff like rocks/ground etc.
If you're doing a lot of street photography, reportage, documentary, on-the-go travel etc, then I'd recommend Leash. It is very lightweight and can easily be removed and stuck in a pocket.
If you're doing lots of landscape and/or bending down to a backpack, then the Slide (or Slide Lite) will give more security. I tend to just forgo a strap and keep everything in a backpack for that slower type of work. It
These days, the only time I use the Slide is with MF cameras for the comfort of the weight on the shoulder. I tend to have slippy-side-down anyway as I find it annoying to have to keep flipping the strap. Have had a couple of near misses with swingdowns!
