There are numerous threads here that have 'soft' confirmation that he will appear as a YG in Series 1. They also alluded to it 'heavily' on a recent Sports Cards Live. That being said,
If you want 'fun', OPC. Boxes will be more even in terms of value and you get a nice mix of inserts and parallels.
If you want 'ROI', it depends. If you can't move the middle-of the road stuff, I think you are better off with OPC. There's really no expectation there and the focus is not on RCs which puts the value in the rarities (of which you will get plenty). You'll get your return in a base set and a few individual rare cards. I think to maximize ROI on Series 1, you have to look at grading the YGs. As was said, once epack releases, you can get the lower tier YGs for $1 or less on COMC, so you also have to move faster.
If you want something big, it's probably Series 1. Canvas YGs, Acetate YGs, Draft RCs and parallels, I think there's higher protentional for that 1 card.
If you were keeping it sealed, I think Series 1 also might be good for that, too.