When possible, I prefer to use the diagonal for a couple of reasons, the main one is it makes the seams in patterned fabric less noticeable, and two because I feel it is more stable, or at least less likely to unravel at the seam line.
I don't think it matters very much. If I'm just short and don't have enough fabric, I'll use a straight seam.
I also prefer to miter border sashes but sometimes that can look weird when all the other seams are blunt and plenty of patterns call for blunt sides even if there are diagonal seams in the block...