Usually, it's top over mid, for a multitude of reasons:
- Top is tanker, and has higher base AD than mid, meaning top can take a hit or two, and the monster goes down faster
- Top has higher odds of starting with a CC ability than mid, meaning less hits taken overall to both leasher and jungler.
- Top usually doesn't use as much mana as mid with their first ability, meaning they're set behind less resource-wise if they leash instead of mid
- The minions start fighting in mid about the same time the monster spawns, limiting your effective leash to 2-3 autos and an ability, tops, before they have to go back to mid if your mid leashes, whereas top has 10 additional seconds.
- Top usually has more innate sustain, meaning they can recover from their leash before mid will.
- You can reach top faster than you can reach mid after leashing on red-side. Blue-side, it's faster for mid, but you're more likely to die if invaded because of the shit position you're in.
- Mid usually is the one to roam and counter-roam most often, meaning being down a single minion worth of experience is more harmful for mid than it is for top.
There are more I missed, sure, but I got most of them.
Also, all these reasons are why bot is usually the one to leash (they leash faster, don't use quite as much mana, have innate sustain, start CC abilities more often...) - and why most junglers will often either solo clear raptors or something, or just start with their botside buff, even if that buff won't be terribly useful for them.