Congrats on the pull. You've probably already made your decision, but I don't think you can lose here. I pulled a 6* Onslaught yesterday and even at just r2 he's amazing. And although I don't play her much, I know Photon is a great champ also. I think the right decision depends on your roster, what modes you enjoy most (BGs, questing, arena, etc) and (just as importantly) which champ you like playing the most. Not knowing any of this, I'd say that if you're fortunate enough to be in a position where you can easily max out a 7* Onslaught, I'd recommend doing so if the difference is substantial between a maxed out 7* and 6* for the modes you like the most. If upgrade resources for 7*s are difficult to come by then Photon seems to be a better pick if you can readily upgrade her to where she's better than your current 6* Photon.