Tough decision: why I went down
With a Terrence (1536) on the board, giant spawns can be up to Threejay (192). Each of the six plus blocks arrive at a ~17% rate. With that in mind, here's our rank-ordering of choices:
1. Down. The player is best off building along the top row in this screenshot, since most of her material is consolidated in that direction. Since Down keeps the first and second columns anchored, it would slot any large plus blocks right in where they want to be and strengthen the anchoring of the top row. Anchoring along an edge is a more stable position than clustering large blocks in a corner.
In addition, at this stage of the game, you need to have constant fear that a giant will spawn. It is not if, but when. Down best allows the player to flexibly accept larger material--more on that later.
2. Right. It's ideal to keep all your largest blocks along one edge. But if you can't - or don't notice that you could - then clustering your big blocks in one corner is the second best thing.
3. Up. Up is very interesting from the previous position (go back and take a look). Only the fourth column is free, so envision going UUU. By then going L, the player has a 67% chance at creating a hole in the top row where the 3s will combine, in which case she can go U again to migrate the potential giant all the way up to the top row. And even if it doesn't work out, the giant is still in the second row, which is generally fine.
4. Left. It messes up your color matching in the fourth column and leaves the potential giant far from the corner where all the other material is.