On a general level I would take Modric any day, as he is top tier world class in a role/position where there's ironically not too many world class players left in the world. BUT if put into our team TODAY I would take Bale. What Modric does is make others around him much, much better with his sublime vision and awareness of his teammates' positions and opportunities, the assister to the assist, as well as his outerworldy ball holding ability. Dembele has always been very good at, as they do in the sport known as hand-egg, getting the extra 10 yards. While he hasn't got a patch on Modric's spacial awareness, he doesn't make that guaranteed extra dribble and stupid decisions to the same extent, which has led to his great impact to our team.
But Modric won't change the fact that Lamela has very little of the said awareness, and he won't improve that Kane sometimes should make the pass instead of shooting with a defender 2 feet away. Nor will he make Alli better at being a receiver than a creator. What Modric is though, is the Eriksen-role and Dembele-role merged into one player, which leaves another player position open to do, ermm... stuff.
Bale however, is an instant tea with amphetamine replacement for Lamela and Son, and has all the qualities they lack or are not so good at. He's got extreme energy and speed, he's great with both feet and his head, he doesn't make rash decisions or move himself into lack of space, he's very versatile, he's selfish when he should be and unselfish when he shouldn't, his movement and positioning is top notch and, he's great at passing and spotting the open man, and he has a vicious shot. We would be an undergarment-wetting threat from every damn single offensive situation that could occur on a football pitch, now that Eriksen has mysteriously forgotten how to strike a ball from set piece. He commands quite some wages, though.
What the hell, get them both and we would win the Champions League.