There's a 200$ difference between the phones, I'd go with the N73 if you're able to get them for the same price.
The k790a is more of a competitor to the N73 and about 100$ cheaper. Both are really good phones but have yet to experience each of 'em.
Oh, and the K618i > K610 (it doesn't have that horrible navigation knob... I hate that part on my k500i)