Even though they were pretty good and what I used before the T3's, the T2's have the coil and wicking on top. There is a removeable mouthpiece to fill, which became a problem because the metal threading came loose from the plastic/delrin mouthpiece. The T3 has a bottom coil and wicking - less dry hits and less burning, and the mouthpiece is fixed, so no problem there. The bottom of the T3 has a knurled piece that connects to the battery and the tank. You have to fill the T3's "upside down" - iow, remove the tank from the knurled piece upside down so as to not spill any ejuice. One thing you have to watch for those is that when the tight system, via three silicone seals is unscrewed, then you'll get some ejuice out the end of the mouthpiece - just make sure you do this over a paper towel/napkin. You won't lose the whole tank, just what was in the coil area. Make sure all three silicon rings/seals are in place before screwing the coil unit back on.
One other thing that some have noted is that compared to the vision stardust and other clearos/attys, the draw on the T3 can be 'easy'. The fix is to break off a toothpick in one of the air holes in the knurled piece - there are three - should be two probably

I've never had a T3 leak on me but early on I did tear up the one silicone ring 'above' (when turned upside down) the threading of the coil unit - so I threw that one away...... I eyeball the rings/seals each time I refill now. Mostly they stay in place but ymmv on that.
The MT3's have a metal skin - only a cosmetic 'improvement' as far as I can see. The Evod's are smaller volume - also with metal skin and the baby T3's are small like the evod but not noticeably, imo, compared to the T3. I'd rather have the volume. The evod coils units are cheaper - about $1.50 vs. the T3's which are around $3 - again, ymmv, depending on sales and volume buying.... They all fit the diameter of eGo's closely vs. the vivi nova, protank and other carto/tank systems.