I recommend Wolff also. I always slightly overspring my 6906/469/5906/4006 models just in case.
Something to perhaps consider, at least in the aluminum-frame models, is that using a heavier recoil spring means the slide/barrel may be returned to battery with greater force than when using a standard recoil spring.
The slide and barrel are retained on the frame by the presence of the slide stop lever pin, meaning the barrel hits against it and is kept from moving further forward.
Over the years some users of the aluminum models have experienced cracked frames where the cracks started at the frames slide stop pin hole.
Why add more force to the impact between the barrel bottom lug and the slide stop assembly's pin, which is secured in the pin holes of the aluminum frame, than needs to occur when the normal spring is used?
Now, I did once see an original 3913TSW which had developed a very odd crack at the front of the left side of the frame's dustcover. It extended about half an inch behind the front of the frame, paralleling the lower edge of the dustcover's inside frame rail. The owner thought it was a scratch in the frame, but couldn't explain how he'd managed to scratch it. When I examined it closer I determined it was actually a crack.
I called S&W and arranged to send them the gun (cop's off-duty weapon). When they called me back they were puzzled how that particular crack had occurred, and the tech told me he'd never seen one in that spot. He asked how many rounds the gun had seen, and I told him the owner claimed to have fired it approx 12,000 rounds.(He shoots a lot on his property.)
Now, I've seen similarly well-used aluminum 3rd gens run for that long, and longer, without developing cracks.
I had an issued 6906 that I tried to wear out, which I eventually estimated had seen approx 45+K rounds fired through that frame (I replaced the slide once). No cracks, but the frame rails were becoming a bit "well polished" along their edges.

I replaced recoil springs about every 2500-3000 rounds, which was sooner than the typical 5K rounds fired/5yrs of service recommendation mentioned in the armorer classes.
I had an issued 5903 (aluminum frame) that saw close to 10K rounds fired by me, and then it was issued to someone else for at least another 10 years of normal use.
My own 4013TSW has seen upwards of 5K rounds and still looks the same as it did at the 2K rounds point. Once when I showed it to a factory rep (who was also a competitor on his own time) after I'd reached the 2500rds fired point, asking him how it looked compared to other aluminum .40's he'd seen, he told me he owned a pair of 4003TSW's he used for competition.
He said that he'd exceeded 25K rounds fired in
each of his own aluminum-framed 4003TSW's, using factory recoil springs (normal replacement intervals), and that the wear on his aluminum .40 frames didn't look any different than the wear on my "lightly used" (2500+rds) 4013TSW frame. He was also a gunsmith on his own right, as I recall (also being a 1911 and 945 shooter, and having co-authored a 1911 'smithing book), and he told me that my 4013TSW was just barely broken in, and I just needed to keep cleaning, lubing and shooting it (and periodically replacing recoil springs

).
Oh yeah, that original 3913TSW that developed the weird crack? The factory cheerfully replaced it with a new one for the owner, apologizing that they had to replace it with the current one, because they no longer had any of the original cutaway grip frames in their inventory. He said he shots that replacement 3913TSW a LOT (on his own range), and he's long since stopped keeping track of the thousands of rounds he's fired through it. (I also gave him a handful of factory recoil springs for it and told him to replace them at least annually, considering his shooting fetish.

) The last time he brought it by for an inspection it was dirty as hell, and he said it had been running like a top for him. After cleaning it I noticed (could finally SEE beneath the fouling

) that he'd started to develop a couple small chipped spots on his extractor. I fit a new one, replacing the spring while I was at it ... and asked him if he needed any more recoil springs.
Another well-used 3913 belonging to a now retired cop I know was used by him when worked a special enforcement UC unit, and he likes to shoot. I think it dated from the early 90's, according to what he remembers of when he first bought it. I saw his extractor also had some minor chipping (but he said it had never experienced a malfunction). I fit a new one and updated a couple other parts and springs. It's still running strong for him. No cracks anywhere in the frame.
Come to think of it, I also serviced a personally-owned 3913 from the early production period of the model for another cop with whom I worked several years ago. He said he'd bought it when working for a previous agency, and had used it for both off-duty and for weekend competitions for many years. He hadn't kept exact records, but said he'd easily fired more than 12K or more rounds just for his competition interest. The guy liked to shoot (and he was good). He originally brought it to me because he'd started to experience some Skips-DA issues (DA hammer fall occurs too soon in trigger pull, causing light strikes due to a hammer fall timing problem). That might happen with a hard used TDA gun which experiences excessively worn hammer and/or drawbar notches. He also had some minor chipping on the extractor hook, but that's not surprising as guns are used for a long time. I had to replace his hammer and drawbar (just replacing one or the other didn't resolve the problem), and I fit a new extractor. No frame cracks, though.
Sorry I got sidetracked and wandered around in this thread, but I'm relaxed and enjoying my morning coffee.
