Hi, I have a TK40, and would get the TK30 if I could too
In the end it really depends on you, both offer similar output but different UI and battery config.
I managed to have a hands-on experience with the TK30 from a friend. It actually looks smaller than the pictures and cute too. Similar head diameter size as the TK40. Its single body tube is thick and knurling feels good, The tailcap is identical with the TA series but does not tail stand. The body tube extender is a smooth anondized type, no knurling though. The threads looked like they are squared, and changing into different sets of modes felt very smooth and quite easy too. The UI is not complicated at all once you understand how it works. Basically you can have 2 switchable modes at one time, but you can switch into different sets respectively by the smooth bezel twists. The beam for this light is also smooth, but has a small donut hole in the hostpot, even at about 5 meters. I doubt this would be a bother in any applicable situations, The TK40's beam however has no donut hole even at 1 meter.
From my experience, it's difficult to find a wow factor if you experience both. But I would say to consider TK30 because of it's configurable size (with or without extender, less or more runtime), and battery configuration. If you were to use the light everyday and for hours, it's easier to pop 1 or 2 Li-IONs into the charger and back in again, it is also easier to switch batteries if they are drained. Than to have managed 8x AAs all the time, also making sure each are close to matching cells. To add, the light is not that imbalanced at all, the head is a little heavier but it's just a small difference.
Are you looking at other MCE options?