When I play the arena, I play gold most of the time these days. When arena came in, I already had the unupped rares, so I had zero incentive for bronze/silver. I started with platinum because it was worth more, but lately I've found that gold is a lot easier to win for me, and it's still worth a decent amount of electrum, plus you can win upped rares there, and I still need a few of those.
There's definitely WAY more competition for decks in gold, though. My brother is in gold league, and I've seen him get knocked out of the top 500 after like 5-6 games if he doesn't start out well. He's also been in the top 10. Conversely, I'm in platinum league, and no matter how bad my deck has been, I've never dropped below the 300s. I'm not even sure if we have 500 people in platinum league. I'm glad I was in gold league only for like the first 3 days or so of arena before leveling into platinum :p