Martinibelgium has it right on the Hornet; a custom tight chamber often makes a great deal of difference in accuracy. It ALWAYS makes a great deal of difference in case longevity. The case is so small that weighing each charge is worthwhile, as well. A .224 barrel w/1/14" twist gives you a much larger selection of bullets than the old guys ever had, and the bullets themselves are better and can be matched to the target: frangible for little critters, less so for bigger.
K-Hornets have the reputation of being less "picky" than the standard case, but I don't have personal experience of the K-variation.
.25 Hornet is a better choice if you intend to casy your own bullets. Don't have any experience with the .270 REN; I think it is a later development than you are considering anyway.
