Which mining of Monero is more cost efficient (both use same expensive energy) CPU (INTEL CORE I3-6300T LOW POWER TRAY PROCESOR - CM8066201927004) or GPU (Nvidia 750Ti)?
I know GPU mining gives you higher hashrate but it also uses more energy.
I do not have data on the i3-6300T, but the AMD FX 8350 also returns about 250 H/s (source), and this should be a faster processor than the i3-6300T (I'm guessing 150 H/s). The i3-6300T uses about 30W (source).
Use the Monero mining calculator here to determine which is best for you based on your energy cost.
The most efficient way to mine Monero now is using AMD Radeon Vega 56/64 cards.
Overclocking the HBM and undervolting the card while runing Cast's XMR miner gives about 2 KH/s for about 150W.
An i3 6100T will net you around 60-80 H/s on xmr-stak-cpu optimized by fireice with settings tweaked which is neglegible as most pools pay out at >0.5 XMR.
According to MoneroBenchmarks, the i3-6300 does 150H/s at 65W and the Nvidia 750Ti does around 250H/s at 60W. So, seems that at the same energy expense the GPU card can give you definitely more hash rate.
Update: AMD Ryzen CPU and AMD RX 550/560 GPU cards can mine Monero with high hashrate and low power consumption. Also RX570/580 and VEGA cards have nice rate/power ratio. For building a cheap rig with a decent hashrate and really low power consumption, AMD RX 550/560 GPU cards are the most used solution.
Check this answer for more details about which type of hardware is the most efficient for mining Monero.