The advantage of using a 70g over a 55g is easy at that range and thats wind, energy, and remaining impact velocity to make the bullet reliably open up.
Now if your pushing the 55g hard, and the 70g easy, then it will probly wash out, like you have noticed, but if you were to get that 70g up to where it should be shooting, 3500-3600, then you will easily see why you would go with the 70g at longer ranges then the 55. It is less effected by wind, and it hits harder, and I'll bet its going faster then the 55 by the time it gets to 400-500 yards, from there on out, the 70 will perform better in every aspect.
The other advantage to using a 70g bullet is close range shots if you hit bone. The 55s are going to fast and the bullet is to light to hold up when they hit virtually anything, they are like a bomb and they just implode, they dont penetrate for chit. The 70g will give a bit more insurance incase you hit a shoulder, that way the coyote will at least have 1, probly 2 broken shoulders with an exit hole, whereas the 55g splashes on the shoulders at times and then you see them running off on 3 legs and wonder what happened or blame the bullet your using, which is typically the case.
I use the 55s from my 243 as a truck gun because they shoot so fast and flat out to 400 yards. Dont have a lot of time to range them, and most the time there already running mach 10, so I choose to use the fastest/flattest for a truck rifle. I've shot plenty with the 55g NBT at around 3800-4000fps, depending on loads. I've experienced splashes on shoulders out to 200 yards or so, on the other hand, if you slip the little 55g behind the shoulder inside 200 yards, you will get great results, but rarely an exit. SOmetimes you get a gaping entry wound, sometimes you get a gaping exit hole. It all just really depends on where you hit them, and how fast the impact velocity of the bullet is when it does hit them.
For a longer range coyote bullet in the 243, I would shoot at least a 70g bullet at the bare minimum, and the 80-87's would be even better yet, or the 95g NBT. The 87g vmax with a BC of .400 shooting 3200-3300fps is about as good as one can get for a LR yote/varmint round in the 243. Less wind deflection and more energy. They dont shoot as flat, but thats the last of your worries at long range. Drop is easy to calculate, wind not so much..
If you really wanna reach out and touch em, try some 105 amax's with a BC of .500 at around 2900-3000fps. That will lay em out to as far as you can hit them. I worked up a good load with these in my stevens and its shooting 1/2-3/4 MOA at 100 yards. Ran out of bullets before I could do any LR shooting with them or shoot any coyotes though.