I'd prefer that trilogies and series were the exception, rather than the rule in genre fiction—truth be told. But if you just
can't trim the fat enough to fit it all into one book, then I'd have to vote for all at once... especially if it's all written already. I'm a reader, not a publisher, after all.
Covers; I don't give two hoots about. I
may look at them once, but usually not at all.