Depending on where you need to go, it doesn't really need to be DL (for ST) or AA (for OW) that you fly on. Given that you expect to fly internationally, on ST you might do a lot of travel on AF, KL or KE, while on OW you might end up flying BA, LA or JL.

So look at the two alliances' coverage (especially relative to your expected travel patterns!) and how their programs look (upgrades on partner airlines? probably not). But the hard product doesn't need to be that of AA or DL, you will have other choices in each alliance.