I've used the Roberts for years and is one of my favorites. The RL-19 is one of the better powder choices going. For coyotes, the ballistic tip is a good choice. So is the Sierra mentioned above. If you're shooting for pest control or bounty, most any of your choices are fine. If for fur, then use something fairly frangible.
I use various brands weighing 115, 117 and 120. The 115's are the ballistic tips. Great for coyote. The 117's are Sierra and Hornady. The 120's are Hornady (old) and Sierra. I have also used the 100 gr. Barnes and they work like a 120 when they open up.
I grew up around the Roberts and cannot seem to get enough of the little guy. You do need to watch your rate of twist for bullet weights. Some older rifles chambered for the Roberts have a 1-12 whch works for the light stuff but not with the heavy. I think I have that right, my memory is shot when I'm tired. And of course a personal little addendum; the Roberts is and always will be a more versatile cartridge than a .243/6mm. The new Partition 10 gr. for the .243/6mm evens up the odds quite a bit, but you can get one for the .257 as well.