We have both. In the Toto line, there is a little lip on the Drake, for example, that keeps "liquid" from running under the tank, so it's not appreciably-harder to clean behind the seat than the smooth lines of a one-piece.
Frankly, the biggest issue in my mind for cleaning is the area around the seat hinges. On our two-piece Drakes, I have the Mayfair seat with the ez-detachable hinges, so you just push the little clips on them to the right and lift the seat off for complete cleaning. The Toto SS114 seat is set up to be pretty-easy to clean (and it's a nice seat, nicer perhaps in a lot of people's minds), but nothing beats detachable hinges for easy cleaning.
Both the one-piece and two-piece are identical in terms of ease of bowl cleaning. What everyone is talking about is just that area behind the seat where the tank intersects the bowl.
As for leaks between the bowl and tank, that is almost a hypothetical advantage in my mind. Because Terry and your plumbers get called out when things go wrong, they probably see a lot more go wrong than the average user ever will. With seven toilets in the house, in 50 years I have never seen even a drip from between the bowl and tank, and some of the toilets we have are from the 1950s.
Installation of a one-piece is a little more unwieldy just because the combined fixture is heavier than either a bowl or tank, but it is easier because you don't have to spend time properly-attaching the tank to the bowl. And handymen-types (and some lesser plumbers) sometimes don't take the time to make sure they leave the job site with the tank nice and firmly in place. But if you hire someone with pride and skills to do the job, it's almost a non-issue.
In my mind, get the combination of performance, looks and price that works best for you. And consider the Toto line, from the (original) Drake (CST744E and its ilk) and Entrada at the value end of the scale to the Drake II (a good value as well) to the very-fine Soiree and Ultramax II at the more expensive end. They all will flush exceptionally-well, and you will find that their quality (and quality-control) is unsurpassed.