Overall specs of the Ace is better including processor & camera (5mp with flash vs 3.15 mp). No info is available about the RAM capacity of the Ace. Also the Optimus records better videos & has bigger battery.
I will recommend that you wait for a few days till Samsung officially announces the details of Ace.
Even if you go ahead with the purchase, you won't be disappointed with any of them.
Ace is worth the extra 3.5k for those who wants android + proper camera i.e. with flash.
if you want Ace just for the sake of having a bigger screen, once look at the resolution & than look at Defy. Ace for ~14k is a best buy anyway over LG O1.