Well first of all, many of us fly it because it's our favourite ride, not because it's the best. In fact, I enjoy the challenges of flying a ride that isn't really the "best" thing at arena combat. I like the Jug because it's big, mean, fast, and has lots of guns. It's the best .50 based gunset in the game, and I've always preferred the .50's to cannon birds of any stripe. Oh, and it's got a stinkpile of ammo too. I can't shoot for beans, but I've got pretty decent E fighting skills, so the Jug just fits my style and my skills. Plus it's a much better looking plane than those tiny Spits and Ponies.
Why is the Jug better than the Pony? Well it's definately tougher and has better firepower. I find it has much superior low speed handling as well to be honest. It's a pretty forgiving bird all around as long as you make sure the Aux tank is empty before you start stall fighting in it. I think the Pony is certainly a better E fighter and it runs better, but that's about it. While the Pony's sustained turn is likely a bit better due to it's better E retention, I'll stallfight a Pony in my Jug any day. It's really going to come down to pilot skill and fuel load in my experience. Sure the Pony is faster, but it doesn't accellerate fast enough to escape without giving a Jug driver a good shot at it's tail first. And lord help you if you try to dive away from a Jug.
I also like the Jug because it's not all that common and it's often under-estimated. There is nothing I like better than seeing a SpitIX and knowing the last thing he will expect is for me to turn with him... right before I turn with him.