Janet hi! We're travelling on FF miles (been working the sign up bonus carousel and have nearly 500k miles/points banked) so choice is slightly limited. On the workable departure dates we could have flown a FF award from Chicago to Brussels on United or from Seattle (home) to SLC to Paris on Delta. Even though we wanted to start or trip in Ghent we chose to fly to Paris for several reasons.
First we save the $600 we would have had to pay to get to Chicago for Seattle. This isn't such a big thing as I'm happy to pay for convenience.
But also we very much prefer flying the long leg of a flight to Europe from starting in the western US, not the east. Since we have piles of FF miles we fly business class, and having more time on the plane (11 hour flight from SLC vs 8 hours from Chicago) to us has proven to be a big advantage in beating jet lag quickly compared to less time. on departure day we're up at 4am regardless. In an 11 hour flight we have time to relax, eat dinner, then lay the seat flat and go to bed for a seven hour sleep (and our kid sleeps 8 hours or more). We wake at 9-10am in Europe and have 90 minutes to have some breakfast and coffee. We get off the plane really quite well-rested. On this type of flight we're usually completely over jet lag after the second night in Europe.
On the shorter East Coast flights though we only get 4-5 hours to sleep, wake at 6-7am Europe time, feel very tired walking off the plane, and usually don't get over jet lag for 4 nights.
Plus United business class is the worst (it's at the end of its life cycle and being upgraded but not soon enough; Delta is very good, very comfortable bed if you don't mind a bit narrow, and configured to be far more private than United where you feel like you're tent-camping with strangers.
So anyway that why Paris to start. If I ever run out of FF miles I will cry bitter tears as I (once again) suffer in coach!