It's much cheaper to do the motor upgrade and if you have good speed stability I don't think the performance would be that much greater.
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Do the motor upgrade. I would guess a planar 2 with the new motor would be a better deck (from the point of speed stability) than a planar 3. I put the new motor on my planar 3 (when I moved from the UK to the US) and it was a very convincing upgrade.
Also search the archives for re-oiling the bearing as oil level makes a huge difference.