I'd be very tempted to make this trade as Seguin is a proven commodity whereas Tarasenko is not. Its a limited keeper, so adding as asset like Seguin should not be ignored. Your draft picks, while quite nice 1st rounders, may never make much of an impact, so letting them go shouldn't matter too much. If it was a top 3 pick, different story.
I'd try to sell him Bernier instead of Markstrom and push on him that Seguin probably won't be that much better than Tarasenko. I'd also try to sell him harder on the idea that this is two pretty good 1st rounders.
Even if he won't budge, having Bernier as your goalie going forward is not that bad an option as he was rated very highly until Quick decided to be a superstar goalie.
I'd do the trade for the assets you get to put into place now as I think it gives you a greater chance of winning the whole thing.