I enjoy both, so it really comes down to value. I've never owned any of the "big three" fan layouts, so when I get a chance to play them I really enjoy them. Based on reviews of people I trust, they tend to get a bit stale when you own them. For me Monster Bash plays the easiest, but it's also the game I'm the least familiar with. I really don't know how deep it plays. It had worried me that people said that it wasn't super deep, but if it's on the same level as Congo, I'd be less worried. I've owned Congo for a few years now and agree that there are things I rarely see. There is quite a lot going on with the diamonds, maps, and mysteries. If you can get past the theme, I feel there is no question that Congo is the better value.
Quoted from jgentry:
I don't know man, maybe I'm just good at that game. There sure is plenty that I suck at though. I've had at least 10 games on Congo over 4 billion though and one over 8 billion. Just plays easy to me and the hardest shot in game (the right ramp) isn't really implemented so that it's needed to advance. The map modes are easy as that is an easy left ramp to make over and over.
You don't need to shoot the right ramp a ton, just a few times for the extra ball. My previous strategy was to hit the left ramp to collect mystery and maps, however recently, I've been going more for grey followed by either combos or caves. My scores seem to be slightly better with this strategy, but it's not like it's unbalanced. There seem to be quite a few ways to play the game.