A warbird and a 3D plane will both be relatively big steps from the apprentice. But very different ones, for a warbird you will need to learn to handle the higher speeds, energy management and if its one with retracts or undercarriage, be able to pull off more difficult landings. Those light 3D planes will open a very different window on the hobby, but they remain slow and fairly forgiving, in that sense, easier perhaps. I guess it depends what interests you most, personally, but a warbird may be a more natural progression. It makes more sense to move to 3D once you are comfortable doing regular aerobatics, including things like inverted flight.
But like you said, you'll probably end up getting both
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afterthought: you may want to consider something like a bixler or an AXN too. It has the speed, but remains very forgiving, allows you to improve your skills and fly in more wind. Also allows you to try FPV and the like should you want to one day. And you may get a taste of soaring.