as said before photoshop is far the best, althought the most difficult to use, mainly for 2 resons (if you just want to make textures)
1: it has (6.0 or higher) vectors, that are resolution indipendent, and that you can use AS layer masks or that you can cover WITH layer masks, and that are an extremely powerful tool if you want to make geometric paint (panels, wires, mech elements, text, paint stripes and so long)
2: the blending options that can be easily activate/deactivate (special effects that you can apply to a layer, bump mapping it, filling with solid colors, gradients, patterns, setting shadows, satin and other stuff in a lot of ways...i know that almost every program can do that...photoshop just do it better in an easyer way)