I fought the 18-20 battle with my Cadenza, I test fit/drove 19s and 20s, and while the rim gets bigger, the tire gets smaller. You feel so much more of the road with 20s and a bit more with 19s. I settled on 18s, and just lowered the car to eliminate the wheel gap.
Ultimately I settled on 18x7.5 wheels so I could tuck them more than the stocks allowed. I run 245/45/R18 tires on the new wheels, which is identical to stock, even with an aftermarket coilover setup, I maintained the smooth ride and couldn't be happier. Even now with winter and the stock suspension back on, the 18" wheels look good, and come spring, the wife has given me the OK to put the coilovers back on
It's all up to you, but my experience say 18s run the best on the Cadenza (even the stock 19s that came on the chromed out Cadenzas are rougher than my 18s)
Stay Humble.