Depends on your anticipated needs and how an extended capacity (for model) works with your methods of carry/concealment.

For example, when I carried an issued 4513TSW, I sometimes carried an 8-rd 4566TSW magazine as a spare instead of the 7-rd 4513TSW spare. Ditto when I carry my CS9, meaning often carrying an 8-rd 3913 magazine instead of the CS9's 7-rd magazine.

However, when I carry one of my G26's (10-rd mags), my spare mag is a stock 10-rd mag ... even though I legally own a G19 mag (bought as a peace officer). Why? Well, mostly because I prefer the way the shorter 10-rd mag doesn't dig into my side, and how I can slip a 10-rd mag into many of my jacket pockets. (Yes, in addition to having invested many thousands of rounds of range time utilizing belt mag carriers, I've also included jacket pocket carry/use of spare mags into range training & drills.)

Another aspect which has influenced my thoughts can be the particular model/caliber of pistol being carried.

For example, I've seen mixed success when it involves pistols chambered in .40 S&W.

Sure, the use of the next larger model's higher capacity mag "should" work, but does it when it comes to any particular combination of gun, mag, shooter and ammo? Especially when it involves different shooters of different skills and experience? Does a magazine spring last as long in the taller mag when it's used in the shorter gun, which has a faster cycling slide velocity?

I've seen a lot of guys experience feeding issues when using G23 & G22 mags in their G27's, and those issues were typically resolved when they returned to using their stock G27 mags. I've not seen as many problems when G26's (or G19's) were being used with longer factory mags, though. Both groups are small samplings, obviously, but I've heard of other instructors and armorers relating some similar experiences, at times.

I experienced some feeding issues when using a SW9940 12-rd mag in a SW9940c (compact), but not when using the same ammo in the shorter stock mag provided for the compact model. A close friend (another instructor) has experienced the same thing with his pair of SW9940/SW9940c guns.

Personally, I'd examine the use of extended capacity magazines (even factory magazines for the next size up model) on a case-by-case basis, and be more inclined to consider it in 9/.45 guns than .40/.357 guns.

I'd be careful to avoid any risk of compromising reliability just to gain a few more rounds of capacity. If I did opt for using a longer mag in a smaller gun, I'd stick with a gun company's recommendation (and factory mags) and periodically check the longer mag for optimal feeding and functioning in the particular gun. I'd keep a careful eye out for any signs of weakening of the spring, too.

Sometimes, the reason for the expression TANSTAAFL becomes obvious.

Just some thoughts. Not anybody's "expert".

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Ret LE Firearms inst & armorer