In my opinion if you have a space problem, then without rooting there is nothing much you can do, that apps2sd app is useless without root access. Even that app cannot move apps which you can't move from settings menu.
The system apps are not an issue, they are stored on their own rom partition, only their data is stored on internal memory.
There are both risks and advantages to rooting, just like anything else you do. Advantages are that it opens you up to a whole lot of possibilities, you can modify each and every thing about your android experience, remove restrictions which take for granted now, like sms to mms conversion limit, ads, memory, themes, remove useless samsung apps, backup an app in any setting and restore it back to exact setting etc. I use a custom rom, and its way faster, smoother and awesome looking than stock. I have never had any problems while rooted.
The risks on the other hand are basically at extreme you destroying your phone, but that is only possible when you use things that are not meant for this phone, rest all mistakes are easily recoverable through Odin. We keep backups for almost everything, you can have a backup of the entire software to restore to in case of mishaps.
As the two guys above as already said, reading is the most important of all, you ignore something simple and that can lead to a whole case of panic, so read everything thoroughly, all guides and tutorials, ask questions, confirm every procedure twice from different people before attempting and you'll be quite happy with the phone.
I am using CM based custom rock with internal memory extended to 1gb, 160 apps all on internal storage and i have never loved my phone more. Can't even imagine going back to that horrible software samsung gives with this phone.
There is also a 'force apps 2 sd without root' procedure given in sticky threads, you should check that if memory is your only reason to root.
P.S. that ROM MANAGER app mentioned above is a serious NO, it is fatal for the ace, corrupts the bootloader, several people have already lost their phones to that app. We use CWM instead.