I'll go with keeping the current AF1 flying and save the 4 billion we don't have. I'll bet there is still a lot of life left in those 2 747s. Plus Trump can use his own 757 in a pinch.
The current planes are 30 years old. The older a plane gets the more it costs to keep it flying. The longer they are down for maintenance. The FAA and the USAF has strict rules about this. 30 years of takeoffs...landings...flying at altitude wears out an airframe.
Do you know how many MILLIONS of miles and hours those airframes have on them?
And then there is the scarcity of parts for a plane that Boeing no longer makes.
Trump can NOT use his private BBJ 757. For one the Secret Service wouldn't allow it and they DO have the authority to tell him no. Secondly...unless you're really itching for President Mike Pence...the worst thing you could hope for Trump to do is jump in that 757...it's a big ol unguarded unsecured target that some jihadist would just LOVE to blow out of the sky.
You REALLY need to take a step back...take a breath and stop letting your knee jerk of anything and everything Trump get in the way of common sense and logic.