The new DS recer is great. The fit is about the same but with a more regular mesh instead of the harsh stuff that they have had in the past. It also has the Bio-morphic material but I dont think that is does anything. The forefoot is the biggest improvement. They got rid of that wierd crap that they used and replaced it with a more regular outsole, kinda like a paired down version of the 2090. Overall the shoe seems very good and very well may become my regular trainer. It is a very simple very solid looking racing shoe, or trainer if you train in flats.
The Racer ST is wider in the forefoot and more stable (the new DS in slightly more stable tham the old) and the Fila will fit a bit like the DS but I think is higher off the ground (10mm, 20mm) but I could be wrong. Fila also has no Medial post.
If the old DS fit well and the onlt complaint you had was the forefoot than I would say to get the new one for sure. It's a great looking shoe.
Hope that helps.