Portillo's italian beef sandwich is better than Chicago Harv's. Portillo's roll is soft inside, with a nice crispy crust outside -- Therefore, the au jus doesn't seep through the crust and cause a soggy mess ... like with Harv's spongy, hot-dog-bun-like roll. At Portillo's, you can choose sweet or spicy peppers. Harv's only offers spicy peppers, I think. The spicy peppers are similar at both places. The quality of the meat is also about the same.
In general, Portillo's sandwich was fresher ... from the roll to the meat. Portillo's is a high volume restaurant, whereas Chicago Harv's is just a small joint that many people miss as they drive by.
Portillo's italian beef sandwich is $4.75. They also have a "Big Beef" size for $6.59. I think Chicago Harv's price is about the same, but it comes with your choice of side.
Note: Chicago Harv's is still worth checking out if you live in Fullerton -- They have hot dogs, pastrami, chicken/beef bowls (the white rice is on par with most Japanese restaurants).
Portillo's (portillos.com) is in Buena Park, a few blocks from Knott's.