I think it depends on where you see yourself ending up after all upgrades that you choose to make. Do you think you will eventually get both the Triplanar and Gavia TPI platter? If so, I would start with the Triplanar since you will lose more money swapping out tonearms than upgrading your platter through Thom. But if you think it is more likely that you will stop with what you have, then I would suggest getting the better platter first because it will have, I am pretty sure, a bigger effect on the sound.
I started with a Galibier Gavia base with the PVC platter (now called the Serac) and a Moerch DP-6 with 12" arm wand. I went through two platter upgrades, first to the aluminum/Teflon/lead shot platter which is no longer available, and later to the current Gavia platter with graphite top plate. I also replaced the Moerch with a Triplanar VII and Stelvio armboard. In my opinion, the upgrade to the Gavia platter with graphite top plate was a bigger improvement than the tonearm upgrade. So if you could only do one, I would go with the better platter and a lesser tonearm. If you are like most of us, though, you will want it all, and in that case I don't think you can go wrong with the full Gavia setup and Triplanar. Good luck!
Dave
I started with a Galibier Gavia base with the PVC platter (now called the Serac) and a Moerch DP-6 with 12" arm wand. I went through two platter upgrades, first to the aluminum/Teflon/lead shot platter which is no longer available, and later to the current Gavia platter with graphite top plate. I also replaced the Moerch with a Triplanar VII and Stelvio armboard. In my opinion, the upgrade to the Gavia platter with graphite top plate was a bigger improvement than the tonearm upgrade. So if you could only do one, I would go with the better platter and a lesser tonearm. If you are like most of us, though, you will want it all, and in that case I don't think you can go wrong with the full Gavia setup and Triplanar. Good luck!
Dave