Sequence-independent vs. sequence-dependent series: Which is better for marketing?

[Quantum]

Sequence-independent series
Sequence-independent series are books in the same world and many times the same characters but each book is more-or-less standalone. For example, Nancy Drew: #1-64 quickly comes to mind. I believe that Storm Front would also qualify as a serial.

Sequence-dependent series
A sequence-dependent series are books in ths same world and many times the same characters, but each book has to be read in a specific sequence to be understood because all them encompass a time- and plot-based story arc. For example, a trilogy or the Harry Potter series).

The question
Which one do you think would be better for marketing? Of course, if you already have a trilogy or other series in mind and fully developed or you want to, then enthusiasm would seem to take precedence. But what if you are just starting out?

It would seem that because a reader could read any book, then a sequence-independent series would be easier to market.

But what kind of thinking has to go into a sequence-independent series as opposed to a sequence-dependent series? I'd assume that a similar amount of world-building would need to be completed before starting the series. Is it more character-driven? Would you have to space out the books more chronologically? Each book would need to end pretty completely, correct? Would there be much "character development" beyond the first book?

[Ian]

I think it depends whether the first book in a series sells well. If it does, I think marketing the rest is easy. Look at H Potter. But then what do I know? In one of my novels one of my characters thought he was the worst salesman ever, and I suspect that was written with myself in mind.