I don't particularly care one way or the other on 18" versus 20" wheels ... - 18" tires are probably less expensive to replace.
As for the 18/20 inch rim decision, the Pacifica Limited trim already upgrades the standard 17 inch wheels to 18s and going to 20s is a $995 upgrade (half the cost of the Uconnect Theater or SafetyTec package). Both options are absolutely gorgeous though ... aesthetically a blonde vs brunette decision IMO. You might find the 18" rims slightly easier to hand wash if that matters as there are no hard to reach sharp angles or crannies for brake dust and grease to collect in so a standard cleaning mitt is fast and effective.
2017 Gas Pacifica Limited, Vel Red/Black/Alloy
Tow, Vac, Adv SafetyTec, UConnect Theater, 18" Wheels