In this context, "a wash" means that the positive and negative aspects of one configuration vs. the other pretty much even out, with neither being that much better, or worse, than the other. Properly executed, both work well and are viable, the differences are in how they affect systems installations and structure (and the weight of those) and, in turn, how that affects aircraft safety in the event (rare though they are) of an un-contained rotor burst on an engine.