NF A14 is standard 140mm fan and it has better airflow and static pressure than NF A15 fan. NF A14 also has more noise and higher max RPM.
NF A15 is unconventional fan, basically a 150mm fan. Due to it's unconventional size, you may not able to fit 2x NF A15 fans into PC case that supports 2x 140mm fans side by side.
NF A14 is standard 140mm fan and it has better airflow and static pressure than NF A15 fan. NF A14 also has more noise and higher max RPM.
NF A15 is unconventional fan, basically a 150mm fan. Due to it's unconventional size, you may not able to fit 2x NF A15 fans into PC case that supports 2x 140mm fans side by side.