Alternate picking

Don’t overthink it. If something is easier to play starting down, start down. If something is easier to play starting up, start up. If it doesn’t matter, it doesn’t matter.

Learn to do it both ways.
Reason is you’ll end up like me lol. I can play runs up the fretboard but once I go backward and start with the upstroke I struggle. So I’d say try learning both ways itll help you in later situations.

Yeah what they said going by what feels good to you is always a good thing man. But one thing to keep in mind is that a lot of runs you might run into when learning solos or something are in a way designed to start on downstrokes to make switching between strings easier. Like how pentatonic runs are easier because they are mostly 2nps and switch after upstrokes and you can travel from string to string using downstrokes after upstrokes which you’ll likely find is easier than switching after downstrokes which will require you to hit the string with an upstroke. Also if you want to be able to alternate pick really fast I would strongly recommend Troy Grady’s series on youtube “Cracking The Code” where he talks about pickslanting which is like a secret shred technique that all the pros use but developed by accident. Alternate picking takes time, in my case at least, but just stick with it man and good luck!!

Hello.
Just like Isaac said, learn what pickslanting is. It might seem weird and unnecessary, but it’s a logical secret that is getting more popular amongst players all the time. The top pro players use this, but many of them don’t know they are doing it. Many figured it out with hours.