CC all the way.
My #1 biggest problem with the EC is that it's trying to be World of Warcraft, and Ultima Online IS NOT World of Warcraft. How anyone thinks that a 125-slot container is "convenient" is beyond me... would make more sense to me if they broke up our "backpack" into five 25-slot bags so that at least I could control where my stuff is and goes based on a typical slotted pack system.
However, I prefer the CC/"Legacy" containers, because to me, they are more immersive (and immersion in the RP of Sosaria is precisely why I still play UO)... the interface had a style to it that said, "Hey, you're in a fantasy world," not "Hey, you're in the grid, enjoy tossing everything into any one of your 125 slots!"
Which leads me to the issue I have with the EC: The legacy containers STILL DO NOT WORK PROPERLY.
Items don't z-axis according to whether you picked them up and dropped them, they z-axis based on some arbitrary EC setting. Which is why if I have sandals and a robe in my backpack, it doesn't matter that I just dropped the sandals into the backpack, if they are dropped onto the robe, they'll "appear" under the robe when I let them go.
The sensitivity of the pointer is also waaaaay off. It's so bad, I'm not even sure you could call it sensitivity. In the CC, if I see five pixels of an object, I can pick it up from underneath whatever is blocking it (hell, I can do that if I see ONE pixel of an object). Not so in the EC. Something prevents me from doing so.
This doesn't include the lag that happens if more than say 10 people are on-screen at any given time.
This doesn't include the weird skip/lag/whatever as you run through customized housing areas where it takes the client a second or two to catch up to loading stuff.
This doesn't include the innumerate other reasons that the EC is subpar.
I'll admit, I WANT the EC to be usable. I've wanted a replacement for the CC forever. Truthfully, the 3D client was that client if ever there was one, but they time they got it working respectably, they pulled the plug on it for the Kingdom Rehashed client.
Yes, there are a great many things the EC does well or better than the CC, but until the PLAY experience matches that of the CC, as far as I'm concerned, to paraphrase South Park, it can go to hell and die.