Ok. So I already know that FHA requires minimum 620 score (some 580, but what does conventional generally require as a minimum?
Also, even if you've got the scores for conventional, what's the incentive if you still don't have the 20% dp? I could be wrong, but seems like, if you don't have the 20% dp, FHA is the easiest route.
TGIF everyone! 
FHA has a standard interest rate, conventional is based on your credit score. FHA requires mortgage insurance to be paid regardless of downpayment (i believe for 5 yrs). If you put 20% down on a conventional you don't have to pay mortgage insurance.
There are also mortgage limits with FHA (varies by area) so if you want to buy a more expensive home than FHA will approve, you'd have to go conventional.
Before the mortgage crisis there was more room to "play" with a conventional loan... such as the no money down types. They still exist but just barely For a lot of people FHA makes the most sense right now.