The standard truck-- essentially similar to those use on earlier EMD switchers and switchers from other builders-- was cheap, and (since most railroads had switches equipped with it) unproblematic from a repair/maintenance standpoint. It is a simple design, and lacks features (swing hangers etc) of trucks designed to give good riding qualities at road speeds.
The Flexicoil truck was an extra-cost option introduced by EMD in the 1950s (not sure exactly when: a reasonable number of SW1200, including the New Haven's, which were built (I think) in 1956 were equipped with it), allowing higher speeds, and so making the locomotive suitable for road-switching duties. (Canadian SW1200 with Flexicoil trucks and larger-than-standard number boxes are often referred to as "SW1200RS".) Some railroads (RF&P for one) incorporated Flexicoil-equipped switchers into m.u. lash-ups for mainline freight service. (For low-speed local and branch-line freights, even SW1500 with standard trucks could be used: one of the model railroad magazines (I think "Railroad Model Craftsman") published a photo of a Conrail SW1500 in m.u. with a GP38 on a local freight to demonstrate that there is a "prototype" for such a lash-up.)
The two truck designs have the same wheelbase and are interchangeable: SW1500 were sometimes operated with one of each! (Photo illustrating this in some versions/editions of the Kalmbach "Diesel Spotter's Guide.")
Another way of getting a unit suitable for road speeds would be to use the Blomberg truck used on EMD F- and GP-series locomotives. Near the end of SW1500 production (1972 or 1973), EMD built an order of "SW1504" for Mexican railways: essentially an SW1500 on a slightly elongated frame (to accommodate the longer wheelbase trucks) with Blombergs. In 1974 the SW1500 was replaced in the EMD catalogue by the MP15: again basically similar to an SW1500 with a further slight frame elongation, and Blomberg trucks as standard equipment. Since the lower-horsepower SW1001 was unlikely to be used as a road engine, this probably meant the end of Flexicoil truck production.