Ive done both fw and sw. I do think sw is easier today than in decades past. I think in alot of ways sw is easier. However, im saying that after someone is very competent with understanding some basics. Water chemistry, bioloads, fish needs, proper maintenance techniques, ect...
I think a fw aquarists with years of experience can certainly move to sw and be successful.
Everyone is different. If your new to the hobby it really comes down to how much time you can put into educating yourself. And your ability to comprehend and retain that knowledge.