I have Stern, DE, Sega and BW. All mechanicals are similar in most cases.
I have found Stern parts are way more expensive than BW. Look at flipper parts - more expensive from most stores for Stern bits (normally 2 time the price!), but when you look at the components, plunger is exactly the same as BW, coil stop is the same (some are wider, but still easily mount the same), the crank is different, but in most cases the original can be reused. Things like pop bumper skirts and rod and rings for pops are the same in many cases.
I find it good to have a bit of variety, plus you get more experienced at diagnosing problems on different machines when an issue pops up.
