first of all make sure it's the tyres fault(most of the time it is),i had a tyre which kept losing pressure,but when i removed it from the rim and put it back it was fine.sometimes the tyres don't fit properly to the rim thus it looses air pressure.
when it comes to tyres i go for the best brand i can afford,as a biker,i know the importance of having the best tyre possible,because on a bike the differance between a good tyre and a bad one is very clear,it has saved my life many times...the same goes for cars....of the two you have mentioned go for the nankang.
it would be good if you replace two atleast,either front or back,but make sure the treadwear is the same on a pair...put the new one's according to your cars drivetrain(in the front if you have a front wheel drive car and vice versa).but it is better if you can replace all four,and switch them back and forth every six months so they wear out equally overtime.
for a few hundred bucks more you can have a Goodyear GA/GP/GT series...which is excellant - http://www.amwltd.com/goodyear/default.htm