Based on thrust tables the 34kv version seems to be a hair over 1% more efficient than the 40kv version at ~4.7 kw power. Even if it turns out to be a 3% efficiency difference, it would still only decrease flight time by about 1 minute 30 seconds at a cruise if you have a 4 kwh battery. So for someone like me who lives 5000 feet in elevation, would like to use smaller and faster wings, and have tandem capabilities, the 40kv motor would work better for me. If you are someone like Bratwurst who is building E-PPG´s that can fly for 75+ minutes and endurance/low noise is the bigger focus and you don´t need extra thrust, go with the 34kv motor.
I would also like to point out that the 40kv motor has the capablilities of pulling upwards of 260 amps so you would likely need a 300 amp esc, but it would be nearly impossible to push the 34kv motor higher than 200 amps so you could use a lower current 200 amp ESC and a slightly lower current battery if needed which could potentially save you over $800 USD.