I love 5004 (more so than 5204). And yes, I agree 3970 more so than 5970 has the DNA of 2499. The 3970 case stylistically is very similar to 2499 (albeit smaller). I have a different view of 3970 vs 5970. From my perspective, while the movement is the same, otherwise they are quite different. Adding the tachometer creates a much different function and look. More than that, the cases are aesthetically almost totally different.
5970 - larger case, different style lugs, rectangular pushers vs round, etc. For me 3970 has a more understated, elegant, simpler (no tachometer), open dial look. It is classy, refined and beautiful. For me, 5970 has a more a sporty, robust presence.