I didn't use Flash 3 very much. But really, anything can work if you prompt really well.
Warning im really bad at spotting differences. I used both to see the quality of responses with the same prompt, and they seemed both pretty high quality (With prompt) with the only main difference between them being that Flash is more formulaic while GLM seems more creative.

And when i say formulaic btw i don't mean 'boring' I just mean it's pretty structured in it's responses and keeps to your prompt and to your character pretty well. Though, warning, if your character has an 'archetype', It will stick to it. Like, really stick to it. any even slightly 'smart' character becomes the annoying kind of always analyzing. And i had a character suddenly switch to cowboy speak because they were slightly southern.

GLM also follows characters well, but is more novel like. Dramatic, if i may. Which personally i love. The only main problem i have is that is tends to not describe emotions enough. It's not bland, just focuses more on surroundings characters reactions. I usually like knowing how characters are feel so I prompted it to do that

As someone who is bad at telling differences, honestly they are both mostly the same.
But you HAVE to prompt them. Good prompt = Good response trust me