Well, I've tried the PTB mod, and didn't like it because no matter what i did it seemed I would pull off a used cart and the filler would stick to the bridge of the atty, pulling it out of the cart and making a mess. I haven't tried the blue foam cart mod.
I would say that the top three most used attys for dripping are 510, 801, and 901... but that's because they seem to be the three most common in use. I'm sure the 401 is up there, too. I've personally only used 901s, as they work with the equipment I own.
Comparing between dripping and carts on the 901, the dripping wins hands down for me. No inconsistent juice feeds from the cart, just a nice wet atty and vaping goodness... though it is inconvenient while driving. I'm usually good for 20-30 minute trips when dripping (I can usually count on getting stuck at a light long enough to put a few drops in when needed), but longer highway drives are when I resort to cartos.