I'm looking a Thailand to Japan return ticket and ScootBiz and Thai Airways economy are about the same price.

Although ScootBiz has a slightly bigger seat it does not have IFE, unless passengers bring their own laptop or tablet (which I will not be taking on the trip). ScootBiz don't seem to offer any better or more food than TG economy, and ScootBiz don't allow non-inflight food/drink. Also ScootBiz don't seem to give ground service priortiy e.g. security queues. So if ScootBiz don't offer anything better than TG economy I'd favour TG since I've flown with them before and been happy and I'd feel more reassured with a 'normal' airline over a LCC if flight delays/cancellations occur.