So i have both now and wanted to post my thoughts for future visitors.
First the tanks. Even though i don't use them now, i have a few thoughts. Vaporesso's tank is smaller, more stylish, easier on the lungs with smoother draw. But it also whistles no matter how open the airflow is. Melo produces more vapor. Flavor wise, in Melo i need to bump up wattage to get a good flavor, which results in an uncomfortable hot vape. It also spits back. I haven't experienced any spitback on Vaporesso's tank. All said, the only reason i wouldn't use Vaporesso's tank is the whistle. Otherwise it is the winner.
For the mods, first of all Tarot has much longer battery life for me. I would say twice as much. I think it comes down to lipo vs 18650 battery differences. It also supposedly allows to use full power all the way until the battery is empty, which i havent tried. On Pico you are limited to your 18650 battery performance. Which gives less and less voltage as the battery goes empty.
Size and weight wise, Tarot is the winner. It is slimmer, lighter, and narrower. But Pico will be a few mms shorter thanks to that battery cap design. Tarot also looks better (subjectively). It also feels much smaller in the hand thanks to its narrowness. Pico feels more solid but i felt like there was a brick in my pocket, Tarot is a bit better in that aspect.
Pico as you already know supports 22 mm tanks only, maybe 23 depending on the tank. Tarot is 22 mm wide too but it allows higher diameters, but it can look funny. But a short tank with 24 mm diameter won't look too bad if the colors match too.
Buttons on the Tarot are better, less rattly but smaller. Pico's menu navigation and mode switching is easier. Pico is also firmware upgradable and has a 3rd party firmware called ArcticFox.
A huge pro for Tarot is the ability to define power curves, from the mod itself. A huge godsend if you are dealing with small error margins on your rebuildable tanks. Or want to do your own temp control on Kanthal. You basically set seperate wattages for every 0.5 second step.
These are what my experiences are. I would go for Tarot Nano anyday, unless i absolutely need to be able to replace batteries. Which i usually do when i go camping for a few days. That is about the only reason i still have the pico.