USDA doesn't require mortgage insurance paid monthly (although there is a financed funding fee paid at closing).
USDA is actually 100% financing, BUT: It is income restrictive. If you make too much money, you won't qualify. The USDA locations are usually so far away from a city that you need a flare gun so people can find you. USDA loans are more liberal when it comes to credit issues compared to FHA.
FHA has more programs available to low income qualifiers. FHA is not income restrictive but it is restricted to a loan size per county. Although USDA is not restricted to loan size, the restricted income limit restricts the loan size.
FHA is 3.5% down. Conventional lending is 5% down, unless you use Fannie Mae's 3% down program. FHA has a $100 down program ($1000 deposit and the difference refunded at closing).
FHA's mortgage insurance payments are stupidly expensive. I used to refer people to FHA but for an extra 1.5% down, might as well go conventional (unless credit score is below 660). Still though, most banks are moving to 640 minimums for FHA, making it harder to qualify for a loan. While there are the few that still do loans with 580 scores, the underwriting process is such that pulling your teeth out with a chisel and hammer is less painful than that process.
USDA's minimum score is 600. Understand that no matter what the guideline states of the entity, the bank can trump it with their own guideline.
Make sure that the loan officer you use is knowledgeable about USDA loans. They need to know if your income qualifies for the area.