FanDuel offers much better odds on sporting events and does so with regularity. DraftKings also has competitive odds, but not to the degree that FanDuel does. FanDuel also has more bet types within each market, but DraftKings excels in every other area mentioned.
What is the safest type of bet? No bet is entirely safe, as you are gambling on uncertain outcomes. Still, the most likely wager to return some money would be betting on a large moneyline favorite. Any type of single wager rather than a multi-leg bet will generally be more likely to cash as well.