I have tried both Army painter and Vallejo, and Vallejo wins hands down. Army painters paints leave a very thick, shiny layer, no matter how much you dilute with water - like the pigment bitz are bigger, plasticky things. I think they're meant for 1:72 scale things and smaller, one quick layer and an equally quick shade (interestingly, Army Painter makes "Quickshade" for just that purpose). Vallejo's paints are equal or better than GW's, just GW metallics and washes are better IMO. The current GW bottles are horrible, drop bottles are far superior. Vallejo Game Colour colours are related to GW colours as they are bright and saturated, Model Colours are more realistic. Model colours might suit your painting style, Sarnath, as you paint over white. They are very popular with the "french school" painters, who also paint over white and use wet palette.
If you have problems with paint separating in drop bottles, you could use shaker balls = little stainless steel balls in the bottles, like in a spray can. Some places sell these online (Green Stuff World I thinks??), but I got mine from an old bicycle's ball bearings and some leftover bits I looted from work - just make sure that they's stainless, regular steel ones will rust.
I heard good things about Privateer P3 paints also, but have not tried them meself.
Btw, both my local independent game shops have had some problems getting Vallejo paints, one of them has even dropped Vallejo due that. Production trouble at Vallejo end, perhaps?
Armored walkers are Slow and Purposefull.