I like the slicer better in 3" as better for, well, slicing (it's a smaller thinner knife, more suited to slicing than, oh, cutting holes in car doors), and Spanto better on the 3.5" because it's a bigger more overbuilt knife, slightly more suited to hacking through car doors*.
* (do not use either knife this way under normal conditions, save that for car accident scenes or the zombie apocalypse.)