It is not an apt comparison.  Siemian got to spend his rookie year on the bench learning from Manning.  Wentz was thrust in as the starter in his rookie year (after playing at North Dakota State - not playing against top competition in college).  In my opinion, Wentz overall has better talent and ceiling than Siemian.  
That is why Eagles are completely committed to Wentz as their franchise QB.  And the Broncos are looking for the slightest opportunity to move on from Siemian.
I like Siemian and I think he can be an effective NFL QB.  But I cannot see him becoming an elite QB (top 10 QB) in the NFL.  Wentz has that potential. 
If tomorrow both Siemian and Wentz are hypothetically available for trade, teams will be willing to give up a high 1st rounder for Wentz.  Siemian will probably fetch a 4th or 5th rounder at best.  Just my honest opinion (and I am a big supporter of Siemian).