That is a really good question! Yesterday I received some LR Boge cartos so I took my dremel and reamed out the delrin caps on an old Map Tank, cut two slots in the carto and gave it a whirl. It works great! Now the problem is that each system has its strengths and weaknesses. Here is what I come out with.
E2 Tank.
Strengths:
Thick, rich vapor. (I've never had a problem with wicking as some folks do).
Better flavor (IMO) than any other device, dripping included.
They last forever with a quick cleaning and dry burn.
Efficient. I use a 3.2Ω version and find I get tons of flavor and vapor at 4 volts so my battery last a long time.
Weaknesses:
They leak. It's just a fact that if you lay them on their side or overfill slightly, you will end up with a puddle of juice. Doesn't seem to happen all the time, only when you least expect it to!
It's getting harder and harder to find SS E2's in 3.2Ω that are R5's (fat wicks). R4's and earlier all suck.
Carto Tanks
Strengths:
Very good vapor. Not as good as the E2 but more than acceptable.
Decent flavor. Again, I prefer the E2's.
They don't leak! This may be the clincher. No leakage whatsoever!
Because they don't leak, you can fill them up all the way. Larger capacity.
Everyone has the Boge LR Cartos in stock.
Weaknesses:
It's a pain to cut the slots in the carto. Not a big deal, but a pain.
God these things are loud! With all the snap, crackle and popping, you can be heard vaping from next door!
Now my main problem with these. In the past I have taken these apart after 3-4 refills and have found that the filler, around the coil, always has some burnt marks. Perhaps being submersed in a tank of liquid this will not be the case anymore. I'll know more after I tear apart a used one. This may not be a big deal but I'm not sure how I feel about vaping burnt filler albeit it's only a small amount of burning going on.
So I use the E2 Tanks at home where I stand up my device when not in use and the Carto Tank when I go out!
That's my take on it. Hope it helps.