I'm kinda switching between 3 and 5 pieces from career to career. I really kinda like the 3 pieces for "when i headcap a mech, I want to have it", but you are stockpiling on mechs quite fast. So going stingy salvage rules helps a bit with that, also keeps you a little more on the edge with actually equiping the mechs you get with the stuff you want. I generally make payment "normal", so you sometimes end up taking money over salvage even in 3-part-careers.
The 5-pieces-setting makes progress a little slower, but in the end, the 3-pieces-low-salvage way is more immersive for me. You still end up with alot of mechs, but playing with PH's "rule of skull" that's basically a self-imposed droplimit, you still use your lighter chassis and can't go the full 400-ton-madness on lower-skull missions and flashpoints
The 5-pieces-setting makes progress a little slower, but in the end, the 3-pieces-low-salvage way is more immersive for me. You still end up with alot of mechs, but playing with PH's "rule of skull" that's basically a self-imposed droplimit, you still use your lighter chassis and can't go the full 400-ton-madness on lower-skull missions and flashpoints