1: 3C is a little less blue, I would say. My eyes did not see the DRY 3C as warm but as very white, neutral light. It was good IMO, best tint to me, yet.
2: Normal T6 is the Cool White tint (usually 1A?). If compared to warm or neutral, it looks a bit blue. If looked all by itself, it looks quite white. Comparing side to side reveals more.
3: Normally you subtract around 20% but also reflector and lens eat light. Use UCL lens to get a couple percents more. More important to me is the clear outlook of the lens :)
Tailcap can also have some resistance and driver has some efficiency. Only real way to know lumens is to use integrating sphere to get some reference what is actually coming out.
4: 25% difference in LUMENS is far more than difference by eye. You need about 4x the amount of light for your eye to perceive it as twice as much light.
To be short: a difference like 700-750lm is really "nothing". Very hard to see that kind of differences if not carefully compared side by side.
I would say, that if you want most light to play with, get U2. For very nice white, neutral light, get 3C and if you are an incan lover or like yellow tints or better color rendering, get the warm one.
My new Neutral White - T6 3C dropin came yesterday. It took about 2 weeks.
I put it in an L2P host and tested draw at the tail using a fresh Tennergy 2600...
High: 3.0 Amps
Medium: 1.05 A
Low: .08 A
The driver is blue and is marked V10+.
About 6 months ago I got one of the first batch of 25 of XinTD C8's. That C8 driver looks the same and has the same marking. At that time, some posters questioned whether the driver was any good. I have not had any trouble with that driver.
I turned the L2P on and left it on a table for 15 minutes. The light was very warm to touch but had not failed or changed mode due to heat.
Others have measured 4200-4500hz for the Nanjg 105C. I can also detect it with my camera on low at 5% duty cycle, mostly a useless test/check as long as the eyes cannot see it (but if one struggles can see it in shower water on medium an low). I think it would be impossible for anyone to even pick anything up at 8Mhz.