No. Both are equally hard.

There are other factors that can determine how many books are purchased as well. Your agent (or lack thereof) and how they negotiate with the editor. The advance. Rights being offered. How profitable the publisher feels the book might be. And keep in mind that a publisher might make a three-book deal and do very little marketing, and do a one-book deal with a ton of marketing, and vice versa. How many books has little to do with how they market it. Remember, there's years between books, and whatever marketing they did for book one is long been forgotten when book two comes out. And if book one tanks, they're stuck with printing book two to honor their contract. There are pros and con either way.
I used to write stories with multi-books in mind and those books went nowhere. They just weren't good because I was thinking too big and not giving the reader a satisfying one-book story. I switched to stand alone books, keeping an eye out for any series potential. I sold one as a trilogy. I got one-book offers and three-book offers, so it could have gone either way. (And yes, there are times when a one-book deal might be better than a three-book deal).
My new book ideas are almost all stand alone, though I do have a couple that I think have the potential to make a good series. When I write those, I'll write them as stand alones, but pitch them as a possible series to my agent.
I think the smart bet is to write the best one-book story you can, and it goes bigger, great, if not, you still have a solid story you can sell.