The STW20N is the better MOSFET as its typical Rds is 0.23 against 0.33 ohm... thats why it has the higher current rating... and is slightly faster switching due to the lower gate charge. The only thing to be aware of is that the construction of the STW device gives a higher junction to ambient thermal characteristic so cannot be run at as high a current without a heatsink... STX = 2.9A max, IRFP = 3.5A max. With a reasonable heatsink there's not much to chose between them...