So which is better all things considered, beyond the size?
Excellent both.
The 38 has a slightly higher maximum bright than the 34 but still not enough to make a good HDR.
The 38 has local dimming (absent in 34) but the areas are few, useless feature.
The 38 has a sphere 2.0 light effect (absent in 34), this is also a nice but useless feature in my opinion.
Basically the 2 panels offer the same things in practice, have the same performance, the 38 being larger is certainly more comfortable for desktop use (especially for the greater vertical height) and is also more spectacular in adventure or sports games thanks to larger sizes.
There is one big difference that must be considered before purchasing:
if you play fps in a competitive way, in this case the 38 is not good, for 2 reasons:
-is too big, instead of giving advantage it is the opposite, it makes the management of the shootings more difficult than 34
- it is heavier to drive, you do less fps than the 34, in games fps like COD or BF do 160fps or 120 makes the difference, at 100-120 fps the panel is less responsive.
Being a lover of fps like COD and BF, although I would really like to keep 38, I think that in the end I will return and keep 34, I repeat not for input lag and response time, these are excellent in both models, very close to the monitors TN, but for too much power demand on 38.
I have 10700k overclocking at 5.2 ghz liquid cooled and rtx2080ti overclocked liquid cooled, on warzone with medium details on 34 I can always stay around 130-160 fps, with 38 I am always between 100-130 and the difference is unfortunately seen, 34 it is more responsive.
I will take the 3080ti but I only expect 30% more performance than my 2080ti, so even that will not be enough to keep 160fps stable on warzone, even worse will be with the new triple AAA games that will come out.
38 inches is spectacular but I think it is more suitable for those who use the PC for work and only play single player offline or adventure games, no competitive fps.
I'm still considering whether to keep 38 the same, but in that case I should take a second monitor, maybe 240hz 16/9 fullhd to be used only for competitive fps and for the rest use the 38, but I don't know how much it is worth considering that with the 34 is able to do everything without having to have 2 monitors.
finaly:
if you don't play seriously in fps games, 38 is the winning choice, if you like to use fps and be competitive the best choice is 34.
wait 4080ti to drive provided 3840x1600 in the fps competitive game.