They are entirely different programs/airlines.

WN is good for (probably) less-crappy coach, redeem very few points for cheap flights ($100 one-way flight is only around 6K WN points, whereas DL would want 12.5K minimum), 100% redemption availability if there is a seat, many fewer fees and more flexible, Companion Pass for free flights, all-737 fleet, arguably nicer employees.

DL is better for international, has First/Business Class, nicer overall flying experience (many say), better IRROPS handling in some cases, Hawaii.

With OP's limited flying schedule, I'd think he/she is best off just picking based on price/schedule. Not going to get to elite anyway, presumably.