Depends on the mission, but the normal fractional/charter players are NetJets, Flexjet, XOjet, Travel Management, Planesense, Wheelsup and Jet Suite.
I’m guessing you’re looking at Excels or Latitudes if you’re comparing Wheels Up and NJ’s.
Something to think about, and I’m sure they’ve mentioned it, but larger fleets mean better recoverability and shorter wait times to not fly on some chartered jet you know nothing about when weather, maintenance or duty issues inevitably delay one your flights. For Flexjet or NetJets, for example, those recoveries will often take place in better equipment whereas one and two plane fleets like Wheels Up means it’ll be another Excel or maybe a King Air 350.
It makes things a tad better when the flight goes off schedule, but at least a Challenger 300/350, G or Global shows up to complete the trip.
If you’re considering the Phenom versus KingAir 350 then I would simply say that the Phenom has a ton of baggage space, is quite cramped inside, is very slow and has long legs. It’s an entry level jet, and your expectations should be set there even though the salesmen will tell you it is the cat’s very own ass. We routinely skunked it with performance, and the owner’s often noticed the greatly reduced block times.