It's more common for people to enter open, and therefore more wins come from the open class. There are some advantages to the Am Bred class. First, you get a longer time to regroup before winners. Refill bait, collect your thoughts, refocus your dog, catch your breath! Second, since there are usually fewer entries, you have a better chance of winning the class and being in the winners ring. When you are new, the more ring experience you can get the better. I don't think there is a significant difference in whether you will win from one class or another - like I said, there are more entries in open so statistically there are more winners there.
I think it also has alot to do with how ringwise your dogs are. I started my yellow Ch in the ring at 6 mos and he went from 12-18 right into Open Yellow without missing a beat. My 2 youngest are 21 & 22 mos and both have only been shown once. I will more than likely start them in Am-Bred. If your dog isn't mature and is still puppyish that is also a good reason to do Am-Bred. Just be sure the dog was actually bred in the USA. I know it sounds obvious, but some people who have not bred their dogs, but live in the US have shown in AM-Bred and had to give up points!
I'm showing my 20 month old boy (PWC) in Am-Bred tomorrow in Raleigh (only MY second "real" show with him - but he is an old hand at this).
He is in Am-Bred on Tuesday/Wednesday because we don't like to put him in the same class with his litter-brother who will be showing in Open. And because our co-owner wasn't going to be available for Bred-By.
Choice of which class among those you are eligible for is a matter of strategy and personal preference. Dash and Charlie are very competitive boys - and both are close to finishing - though they are different enough that they have yet to face each other in the Winners Class.