No contest on engines, BIG BLOCKS ROCK! The CAT, will take much better care of that heavy of a rig then the ISL. Our ISL, an older CAPS era ISL370 flashed to 400, gets the job done - but I feel the high power CAT will give you a safety edge on the road via more power.
Good tip to validate the build year of the ISL, and understand if the wrist pins are in it.
As far as the coaches, they are both fine coaches. So interior layout would be the deciding factor here.
Both coaches have good support from companies that have risen from the ashes.
Also be aware that a 45' coach, may require different driver's license in many states.
I would put the Intrigue closer to the Executive then the Dynasty in the pecking order. But much depends upon the year. (And, all a matter of opinion
Best of luck to you on your search!