I prefer a straight eyed hook for any type of pattern. Pure speculation, but I think down eyed hooks come from a time when the fly needed to be snelled onto the leader, a step up from the spade head. Like snake guides they’ve become part of fly fishing tradition. Fortunately, for me, most companies that produce “fly tying” hooks have straight eyed hooks for the various styles needed for trout patterns. Once I get into the larger panfish, bass/pike/muskie and salt water patterns I’ll switch to conventional hooks, Aberdeen style crappie hooks, mosquito hooks, drop shot hooks, straight shanked worm hooks, jig hooks and treble hooks. Don’t sweat it if you don’t have down eyed hooks.
Partly (it is largely due to tradition.) It had nothing to do with snelled flies, but everything to do with the Turle knot, which doesn’t work at all with a straight eyed hook. Since I still use that knot on rare occasions, I’ll stick with the down eye. If you use some other knot, a straight eyed hook works just as well, or perhaps better in some cases.
Typically matters more to the tier than the efficacy of the fly. There are some cases where a straight/up/down/jig eye are preferred due to tradition, the size of the pattern, or how the pattern is presented, but as far as hooking ability, the point is for the most part moot.
I typically use straight eye for sizes 18 and smaller hooks to give more clearance when tying. otherwise I prefer down eyes although I have read from those who have performed unscientific tests that there is no significant differences in hooking chances