wait, u mean Ace is using Mono loudspeaker.
so how the sound quality while listening to music?

Get Galaxy SL if you a hardcore multimedia user, else Ace is suffice.

Except, I wanna know, is there any equivalent size samsung battery with higher capacity for Ace ? I feel the power capacity isn't enough, lol.

Two... one for loud speaker and the other for earpiece. Phone does not need a stereo loudspeaker coz how much details can it goes with those puny little thing? As long as it has headphone jack, it's good already. If I need a good audio output, just plug in the headphone. Mono and downmix is two different kind of thing and phone loudspeaker is downmix.

And how many phone out there comes with two speakers for stereo output?

Did you know that most HD recordable phone out there is still record mono sound coz of one mic 

Welcome. Glad to be in help.
Unless you frequently change ur phone, if not is better to  invest a lil bit more so the phone last longer.
And i'm pretty sure galaxy S/SL will last longer than Ace, let it be performance or future update, samsung high end phones tend to get more update than lower end one.