I've flown them both in the past. The gun packages on the Pony and Kurt have exactly the opposite strengths and weaknesses. The Pony is good for getting some hits at long range and high deflections, yet you may often find yourself landing hits to little effect, especially on short range crossing snapshots or shooting with a very high closure rate. The tater gun will never serve as a sniper weapon, yet if one learns to follow the "damn near ram" philosophy of shooting, it is satisfying overkill that won't let yo down at close range. (The La7 gun package seems to basically split the difference)
Neither is a good noob's plane, the P-51D simply doesn't have any attributes, especially turning ability, that will compensate for offensive/defensive mistakes except the ability dive away. Not very inspiring. The K4 can do much more (including running even faster) but the tater gun is simply a handicap for noobs.