I presume they believe the .357 has more "power"?
I never could quite get my head around how a smaller, yet otherwise identical case, could deliver more "power"?
Ya, off the shelf, the .40's springs are probly a bit underpowered for warmer .40 loads ... more comparable to .357 Sig ... but I'd rather pay $8 for a new 22 - 24 lbs drop-in recoil spring, than a premium on "practice" ammo!
Here's a couple loads I found in about 2 minutes of searching ... 'bout as close in bullet-weight, as I could find ... the 147gr .357 is touted as a "woods load" ... so it ain't like it's otherwise just another FMJ!

http://www.doubletapammo.com/php/catalog/product_info.php?cPath=21_32&products_id=617
357 Sig 147gr. FMJ-FP
Here is the heavy hitter! Recently there have been no options for the serious .357 Sig shooter other than 125gr. This is the best woods load there is for your 357 Sig. At DoubleTap we have developed a loading that combines speed, power, and match grade accuracy! All of this and we use a flash supressed powder for virtually no muzzle flash.
3.5" barrel - 1213fps / 480 ft/lbs
4.5" barrel - 1296fps / 550 ft/lbs
http://www.doubletapammo.com/php/catalog/product_info.php?cPath=21_26&products_id=398
.40 S&W 150gr Nosler JHP
A great carry load for personal protection in 40S&W.
1218fps 496 ft/lbs from a 3.5"bbl.
1317fps 580 ft/lbs from a 4.5"bbl.
So long as the .40 is more powerful, the fact that it's a slightly heavier bullet is of no matter ... but what's interesting is, being a JHP, the .40 is taking up a bit more room ... that could otherwise be used for more powder ... at which, it's somewhat handicapped!
That's not to say that these two loads are indisputable proof that one particular .357 load, in one particular gun, couldn't ever be more "powerful" than another particular .40 load, in another particular gun!

But, it does show .40 can run comparably fast with midrange bullet weights! Doubletap also offers a 135gr load over 1,300fps from a 3.5in bbl ... of course, this is slower than a 125gr .357!
