I think the biggest difference you'll find for the comparable models in both lines is the Shadow Cruiser does not have the front window. I believe that the two are comparable except as LJAZ noted, the FF X Series tend to be a little more "upscale" in decor finish.
You will also find that FF X series has more in the way of ala carte "factory options" than the Shadow Cruiser line and when you get up into the bigger TTs in each series, the FF X line has other "upgraded standard" features, i.e., two 30lb LPG tanks vs. the Shadow's two 20's, etc.
My 2 cents, your mileage may vary...
2 Cats; J-Lo and Ragamuffin :R
2014 Thor Tuscany 40RX DP
2011 Ram 2500 Longhorn CTD HO
2011 Keystone Cougar 318SAB (now gone)
2008 FunFinder X 210WBS (Sadly gone)