I've seen some recipes call for green onions but always use spring onions? Is there a difference between them? Is there a better substitute?
It's a regional preference on what they're called.
When you're buying seeds, they're also called "bunching onions", and I grew up calling them "scallions", although I think that scallion specifically don't have a bulb yet formed, while green/spring onions might.
update : A little research suggests that "spring onion" is the preferred term in the UK, AU and Canada, "green onion" in the US south, and "scallion" in the US northeast, however I'm not sure what the exact boundries are as in the US mid-atlantic, I'll see both "green onions" and "scallions" for sale, with "green onions" being larger (scallions with no bulb, maybe 12"/30cm long, while green onions might have a 2"/5cm bulb, and have over 24"/60cm of green top). It's possible that "green onion" might be a polysemous term that varies by region.
Southeast US: Scallions and green onions are basically the same thing, no bulb; spring onions have a bulb.
Spring onions have a large bulb at the base. The bulb is mild in flavor and the flavor in the stalk is more intense. Green onions have a much smaller white end. The stalk of the green onion is milder than the bulb/end.