Most monolights don't have HSS for outdoor use, some do but the difference in cost may put you off.
Main question - from a workflow POV there's no question about it, mains powered studio flashes are streets ahead in every way - recycling time, power, modelling lamp, all of which are more than helpful.
From a results POV again, they win, simply because the vast majority of speedlights have a fixed, built in reflector. This severely limits the way that the light can spread inside a softbox, which means that you can't get even illumination in a softbox of medium or large size, also it produces very peculiar results with modifiers such as beauty dishes, snoot, silks, scrims and so on - but most umbrellas are pretty much OK