I'd go for Modric.
Nani to be first choice LW, Valencia first choice RW - Modric would then play either CM in a 442 (as he has shown he can do) or as the more attacking player in a 451, offering better support to the striker than we usually see from our midfield. That CM/AMC role needs to be filled this summer, imo, and there are few players I would trust more than Modric to fill it (that I feel we could actually get).
Bale would be a very welcome addition to the squad, but at the price that would be required, it would be a poor purchase imo - as we woulnd't be able to strengthen in the more pressing areas of the team as well (due to costs) which are GK and CM, imo. We don't need to go out and purchase an expensive world class (arguable) left winger as we will be fine with Valencai and Nani as as the first choice wingers, but I do feel we need to bring in someone who can play on the left. Ashley Young, with just a year left on his contract in the summer, would be a much more realistic option in my opinion. Not as good as Bale, but he can be very good on his day and can play either side - would offer very good rotation with both Nani and Valencia.