If you want bbm, yes, only bbm, then it's worth having the 9300 as opposed to the Ace, as the Ace obviously doesn't have bbm. Otherwise, the Ace is better in every single way. The 9300 has 2 megapixel camera without a flash, whereas the Ace has a 5 megapixel camera with flash. The screen quality and screen size too, are better on the Ace dazzling with 16 million colours, at 3.5 inches, and it is as responsive as an iPhone too. As opposed to the 9300 which only has a dull 65k colours and is pixelated. The screen size too is a mere 2.5 inches (although it is non-touchscreen, but you have to take into account that touchscreens are dominating the market)
The Ace also has a better built quality. It has a solid 'corning gorilla glass' screen which can resist good impact from dropping and scratches too. It's not the slightest bit flimsy and has a high quality feel. The 9300 on the other side, has a tacky built quality with a glossy polished frame that peels off after a while. It just looks cheap even though it feels a bit solid. Its screen is flimsy too compared to the Ace. The Ace is newer too being released a year ago, whereas the 9300 has been around for over a year and a half. However, the only thing that is better on the 9300, is the battery life. And even that, not by too far.
Speaking of downloading apps, the Ace being an Android, especially holding a 2.3 android operating system (OS), the android app market is far more fun to browse and use compared to the limited and inferior Blackberry App World Market which plays up a lot and causes complications. The Ace is also very customizable when it comes to dragging or dropping icons around. Most of all though, the photo or video viewing experience is far better on the Ace with its screen size and colours of course. It has a powerful 800 mhz processor too, therefore you won't have any freezing or buffering whereas the 9300 comes with a mere 624 mhz processor (have to admit, still powerful for a non-touchscreen though).
In my opinion you have to be selfish sometimes and can't always think about what friends want from you. This is just one of these cases, so basically if you care about what's best for you, NOT THEM, without hesitation, choose the Ace. If you really care about what your friends want from you, then I suppose you would go for the 9300. But that's your waste of money then for far less features. I'd rather go for the Ace and tell them how much better it is then the 9300. There's nothing wrong with using sms, which so many teenagers have literally forgotten about since the manipulating bbm service became so popular. The Samsung Galaxy Ace is the Winner without a doubt. It gives you far more quality features with a better built quality. It also scores higher on looks.