Two engines. Greater probability of a technical, single engine failure. Some, but not huge, difference in probabilites of single and total power failure through FOD, birdstrike, combat damage, etc - although (again) double engine failure much less likely. Probaility of total power loss leading to loss of airframe, significantly less.
Single engine. Yep, smaller probability of a single engine failure, but when it happens the consequences are in a different league.
Why build single engined aircraft? Cost, weight, size. Fine for trainers or if you want larger numbers of airframes, but if you're building the country's new "do everything" fighter-bomber, logic dictates that two are better than one. It offers greater survivability and payload, although as soon as you go to vertical flying, you've already accepted that payload isn't near the top of your wishlist.