Buying an SLC will give you two styli with very similar charcteristics, so I’d probably advise against doing that. The Shibata on the other hand will provide a little more body and a slightly richer mid range, making it rather less like buying the same thing twice. Neither the ML nor the SLC is lacking in those departments, but rather than being absolutely neutral the Shibata will add a little extra romantic ambience to the final product. If that sounds good to you, then the Shibata is my recommendation for a second stylus.
If/when you want to branch out and try an MC, Audio-Technica has you very well covered there too. The ART9, AT33Sa and AT33PTG/II all perform very well indeed in the SA-750 and either one will have you smiling in no time. I wouldn’t advise moving futher down the AT33 line though, as the AT33EV with its nude elliptical stylus is about on par with the 740ML you now use. To my ears at least it will require an AT33PTG/II to conclusively better it.
There’s also the new OC9X line of cartridges, but to date I’ve not heard any of those.
Finally I’ll just reiterate that to my ears the ART9 is one of the greatest bargains there is, equalling anything I’ve owned or heard.
As always: in my opinion, based on my ears and my system.