Shiny new cheap AR-15, or shiny new cheap 3D printer?
An AR-15 typically won't pay for itself. A 3D printer in the hands of an engineer? Most certainly can, but it's kind of hard to shoot your way out of a situation that absolutely demands Enough Gun to survive with it.
If I don't get an AR-15, I pretty much don't have anything else that can serve its function other than maybe a sub2k (nowhere near as capable, but small and handy at least). 5.56 seems to be the only caliber I can find that I'm particularly interested in. On the other hand, I don't predict a very high probability of need for it quite yet, or I'd already have it rather than hemming and hawing over which to get. I certainly wouldn't count out the possibility of something stupid happening that'll make me want it, of course.
If I don't get a 3D printer, I can't do easy prototyping of a lot of things that lend themselves really well to 3D printing, and can potentially make me all the money. One of them is a new type of magazine for magfed paintball, 30 rounds of first strike (shotgun slug ballistics) in the space of a regular AR-15 mag (most paintball magazines are either ungodly massive, 10-12 rounds of capacity, or very unreliable. If I can make a proper capacity magazine that is reliable, I will probably pay for my 3D printer and a shiny new AR-15 without the price cap in one fell swoop)
And all this indecisiveness will probably be rendered moot as soon as I find a job.