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Both of those bodies have "self-cleaning" sensors, which I have found are very good at shedding dust, fibers, etc.
The only internal source of dirt in the mirror box would be oil or grease flapped there by the mirror, which might be likely on older bodies, but unlikely with newer bodies.
From outside, however, talking, stuff falling in if the body mount is facing up during lens changes may drop something that gets flapped on the sensor by the mirror.
If the spots are water soluble, oily or whatever, key is the appropriate cleaning fluid on the appropriate swab and using it by the book, rather than brute force. I have never had anything on the sensor I couldn't remove (except a scratch Canon put on the original sensor which they replaced for free).
If you are using reasonable awareness while changing lenses, and using body and lens caps when appropriate, I can't see why you would have chronic problems with the bodies you have.