Design and Looks:
Both phones have the simplest of design with hardly anything extravagant and over done. Optimus black is a very slim block with only 9.2mm of thickness and weighing only 109g. Galaxy Ace is more on the thicker side with a thickness of 11.5mm but weighs still just 113g.
Galaxy Arc has the rather old fashioned capacitive TFT touchscreen with resolution of 320 x 480 pixels and 16M color support and screen size of 3.5 inches. Optimus Black features the latest IPS LCD capacitive touchscreen with higher resolution, 480 x 800 pixels and also a bigger 4.0 inch screen. Both the displays are Gorilla Glassed.
Both phones feature a 5MP autofocus camera with LED flash and resolution of 2592×1944 pixels. Optimus Black is able to record HD 720p video at 30fps but Galaxy Ace only records QVGA video at 15fps. Optimus Black also features a 2MP secondary camera unlike Galaxy Ace. Both phones have the geo-tagging and face and smile detection features.
Processor And OS:
Optimus Black features 1 GHz Cortex-A8 processor powered by power VR GPU and runs Android 2.2(froyo).Galaxy Ace has 800MHz ARM 11 processor with Adreno 200 GPU, it also runs Android 2.2(froyo). The OS in both smart phones is upgradeable to Android2.3 (Gingerbread).
Optimus Black has 2GB of internal storage capacity whereas Galaxy Ace only has 158MB of internal storage. The storage can be extended in both phones via sd memory card up to 32GB.
Both smart phones have Bluetooth v1.2 with A2DP and wifi support. There also have the micro USB v2.0 support.