What is the average and maximum cursing speed of your friends? What kind of terrain are they riding?
As a bicycle fullseven 9 is a much better bike than Bulls twenty8 which is just a hybrid with coil front fork and limited tire clearance. Haibike has very good suspension components, Magura MT7 brakes, better shifter/derailleur etc.
Unless you are riding a fat tire the difference between mtb tires and 700c for ebikes is not much. What is going to affect your range will be the speed, below 20mph these bikes are efficient if you ride with avg speeds above 20mph then your range will take a significant hit.
For riding position you can get a quality adjustable stem, if you are riding mostly on pavement then you may go for a smoother tire, for gearing a larger front cog is an option.
A hybrid bike like the bulls offering will never have the grip, control, comfort of a good full suspension like FS 9. While 28mph may look good on paper a delimited CX is actually better than the HS motor on the Bulls. You can always put better rolling tires and make a great commuter of the FS haibike but you can not make a hard tail with limited tire clearance like Bulls come any close to the capabilities of a good FS bike.
I have two bikes which are very similar to both Bulls and the FS Haibike and I would prefer the FS bike to the hybrid every time. When on the trails it is not even close, FS is amazong , when riding at high speeds (happens when going downhill) I feel like FS has better handling thanks to wider tires.