The 3520D has a screw back while the 3919 has a snap back. One other difference is the size of the crowns, I found the crown on the 3520D quite small and not as easy to wind whereas the crown on the 3919 is somewhat larger but large enough to make the winding process much easier.
Try setting and winding both to see what you think.
The 3520D used the Ultra Thin movement which made it about 25% more expensive than the 3919. When viewing the movements, the 3520 had the Geneva seal engraved, not stamped like the 3919 (these observations were made about 15-20 years ago so there may have been some changes since)