The 220 Swift and the 5.6x61R were intended for drastically different purposes (varmints vs big game) and neither one of them won the popularity contest for that purpose. The Secondary Explosion Effect Is not something that is limited to a particular caliber or a couple thousandths difference in bullet diameter. The first I heard if the "SEE" was by Jack O Connor with regard to the 270 Win. At the time, the examples were with the .270 smaller than "starting loads "of slow burning powder (in the 270's case 4831). This has been under contention since because no one seems to be able to duplicate the effect, at will, under controlled conditions; yet there are several reports of uncontrolled cases. Considering the potential costs of uncontrolled incidents, it just makes sense avoid the conditions most people think cause "SEE", unless something else is proven to cause it. It might make more sense to compare 5.6x61 with 5.6x57(big game) and 220 Swift with 22-250 (varmints), while understanding that all four can be loaded for big game or varmints. On the other hand, the choice to buy a particular rifle is more likely to depend on the availability of the rifle and components and especially the individual's particular interests than a comparison between different cartridges.
Mike