Yes, one of the advantages of an 18" wheel is that it's less prone to pinched flats. There's a considerable bit more depth and "meat" in an 18" tire (say, 120/100-18) than your regular moto setup, which gives more cushion for rocks, etc.
IMHO, it depends on how set you are on racing enduro and x-country events. You want the most reliable setup for natural terrain. When you hit some sharp rocks while screaming at WOT, it may have been well worth the investment for an 18" setup with the heaviest tubes available.
If this type of racing isn't something serious and you don't have play cash to throw around left and right, then you could survive with the 19" just fine. But if you plan on making a commitment, run what's most proven and reliable.
Also most tires best suited for trails and enduros come in 18" flavors.
~JuGGz