Both seem to have loyal users but here are the actual UI differences I noticed when using the two. In terms of concrete UI Swype just seems way better thought out. The lack of the first two items listed under Swype in particular made Swiftkey need a lot of extra keystrokes and much slower.
Swype:
++Delete whole word after swiping
++Auto space after punctuation like commas
+Auto capitalize/new sentence after periods
+Dedicated comma button
+Provides close alternate word matches after you swipe instead of autosuggesting the next word until you press space
+Doesn't require you to give it Full Access
+You can manually add words, useful for things like e-mail addresses/etc
-Annoying swype button to access the globe icon
Swiftkey:
+Animation and dark colors looks nicer/smoother
+Learns words automatically
Swype:
++Delete whole word after swiping
++Auto space after punctuation like commas
+Auto capitalize/new sentence after periods
+Dedicated comma button
+Provides close alternate word matches after you swipe instead of autosuggesting the next word until you press space
+Doesn't require you to give it Full Access
+You can manually add words, useful for things like e-mail addresses/etc
-Annoying swype button to access the globe icon
Swiftkey:
+Animation and dark colors looks nicer/smoother
+Learns words automatically