Armstech is probably the worst crew skill you can get if your trying to make money. If your looking for better barrels for custom grade weapons then go for it. But your not going to make much money (if any) on Prototype (blue) barrels although you can make money on the Artifact (purple) barrels but not as much as other professions, in my opinion.
Biotech has too much competition, but if you pay attention to the GTN you can make an ok amount of money on low level implants.
Cybertech and Artifice make the most money of all my crafters.
You can do decent on lower level armor from both Synthweaving and Armormech also.
Of course if your an alt-aholic like I am, you'll have one of each crafting class plus some slicers/gatherers. With all 6 crafting classes plus one slicer, I keep all my characters and companions geared better then what I can get off the drops unless I get a rare purple drop. And even then my blue gear is sometimes better. Plus I can add Augments to all my gear that I make myself.
I'm more concerned with keeping gear upgraded than credits & have Cybertech on my Assassin so should I take Artifice for gun crystal upgrades?