Is it better to reserve flash for a kill or an escape?

an Escape, but only if you know you can actually get away.

You want to save flash as long as possible and hopefully try to make your opponent burn their flash.

Once they burn their flash, then you can think about using your flash to secure a kill because they don't have the summoner spell up.

EDIT:

Just remember, the first person to burn a summoner spell is usually then at a disadvantage. Play more cautiously if you're forced to burn your summoners to prevent a death and hang back until timers come back.

I think it is situational but I definitely use my flash a lot more for kills than to get away. You can use map awareness and decision making so that you aren't in a spot where you need flash to escape. Map awareness and decision making won't give you that gap close you need to finish off a kill. I rather burn the summoner for gold rather than to escape, but this is because I play League with Gold Per Minute constantly on my mind since I believe games are carried based on how efficient your GPM is.

Depends on your champion. If they are mobile, just use flash for the kills because the champion you are playing most likely relies on kills to snowball. If not, then reserve it as an escape.