I mean, I can't remember much about the ranger, but surely if damage is all you care about, then go for the second attack? two attacks, each with ~8 dice of sneak attack, is surely better than one with 9. You get a ability score increase either way (16/4 would get two, but no extra attack, and would miss the 17th level ability).
The 17th level ability would be good, but not game changing. I haven't read scout, but the Assassin's death attack is a little boring. The double damage might make up for having only one attack, even if it is only 1/combat or less.
But overall, I think rogue 15/ranger 5 is better dps than the alternative. I kinda like the second level spells you get from that. A consolation prize, maybe.