US practically has all titles, except Japanese/Chinese language games. SG has very few games, dlcs, avatars, themes, basically everything is far far behind. Generally all indie titles will not make it to SG psn store at all, unless they happen to be offered as PS+ free game (probably arranged by US side). This is not just the tiny indie studios but also including Telltale etc. Even if a title, AAA or indie, appears on SG psn store, very often they are late. Even Sony's own titles can be late, by a lot, or never even appear. After a title gets onto SG psn store, you can also expect that DLCs be late or never appear, even Sony's own DLCs. Hack, Sony's been selling the PS headsets in Asia officially but their headset companion app didn't make it to SG psn store until like 4 years later!
The only thing SG psn store has a lot is DLCs for Musou games (and fighting games). For every game, there are char dlcs, costume dlcs, voice dlcs, wallpaper dlc, bgm dlcs, scenarios dlcs etc etc. Multiple that by 3 platforms (ps4, ps3, psv, esach sold separately). Then multiple that by 3 languages (en, ch, jp, each sold separately). So in a week when a new wave of DLCs hit the store, you can expect to see like 500 or 1000 new DLCs for such games.
Oh and if you are thinking you can buy Chinese versions of games on SG store, think again if its a Bandai Namco title (and some other publishers too, like Sega). Cos they only release the Chinese ver on HK/TW stores, not SG store (while the corresponding EN ver, which typically come months later, even years later in the case of Yakuza games, are not released on HK/TW stores, go figure).
US psn has pre-order and pre-load of many games, especially the AAA titels. SG psn store didn't have this for years, their so call pre-order was basically a free 'ticket' that you buy first which entitles you to a small discount when the game is actually released. There was no pre-loading (pre-loading means the game gets downloaded to your ps before release date and you can start playing the second the release date is reached) on SG psn for ages. It's only this year that SG started real pre-ordering (even though Japan had it for like a year already) but the number of titles still very very few in comparison.
US has sale every week, and flash sales every month (often flash sales are very good deals, like when they have $1 flash sale, $5 flash sale). SG rarely has sale, nowadays they have more but still light years behind in terms of number of titles and price. Just look at the psn store, SG doesn't even have a sale section in its menu, the rare sales only appear in banners. US store has a menu section called "weekly deals", which says it all. Even if they have the same sale, SG has less titles (in a recent sale common across regions, SG had 15 titles on sale, EU had 50, US skip that sale cos they had another going on). Even if they have the same titles on sale, after converting the price to local currency the US price beats SG by a wide margin. SG price only wins once in a blue moon when big developers (like EA and Ubi) put a few titles on clearance.
Talking about price, US store constantly adjusts the regular prices of games as they age. You can expect old games to have a lower price even when not on sale. On the SG store, prices remain forever as the day 1 price even 10 years later. One positive though is that Sony's own big first party games do tend to be slightly cheaper on day 1 on SG store.
Bottomline, US psn account is a MUST HAVE. Even if you don't play using US psn account, you still must have it just for buying the occasional bargain games. (Note if your game is Asia region don't pick up a US dlc even if it's dirty cheap on sale on US side cos very likely they are not cross-region compatible.)