Sergiu,
Two reasons that people go for the nano, the first is the window is much larger. You can more easily see the juice. However, that's not an issue now because Joyetech just released some a new eGo One tank with bigger windows to address the small window complaint.
The new eGo One larger window tank:
http://www.joyetech.com/product/details.php?gno=223
The second is the wicking material. Ego One wicks aren't 100% cotton, but the subtank Nano OCC coils are (the new batches anyway). It seems that Joyetech are working on this though, as their new Ego One rebuildable decks says it's 100% cotton.
If we see them upgrade their normal coils to 100% cotton, there won't be much reason to switch beyond trying a different tank.