Samsung's dual core vs both nvidia's dual core atrix & LG. Which one suits you on both benchmarks & speed
Samsung's screen is larger 4.3" vs both 4" moto & LG
Super amoled plus non pentile 800x480 S2 VS. IPS LCD LG 800x480 VS. Atrix 960x540 pixels lcd. The screens of course.
Battery 1500MAH Samsung & LG VS 1940MAH Moto atrix. Um i don't know the quoted life span, but judging from pure numbers moto obviously has the larger battery, look into this yourself cuz it depends on your needs. All phones have different screens that will be a factor in battery life.
All run Android OS 2.2 stock. LG, Samsung & Moto have their skins on it. Moto Blur, Samsung touch wiz 4.0, and i forgot what LG's is called
See what you like and how it's laid out for you. If you're into customizing, then this doesn't really matter.
Cameras LG & Samsung record 1080p and both have 8mega pixels. Moto has 5mega pixels and records 720p, what suits your needs. All have flash, moto has dual leds though. All 3 have front facing cameras.
Moto has a notification LED, I'm not too sure if LG and Samsung does too. This may be a feature you want.
Build quality seem excellent on all 3. Feel out the phone, do you like light weight with plastic or heavy with harder materials.
See the button layout, and see what one you like i.e Samsung's lock key is on the right side, while moto is behind the phone and LG is on top. Etc
Also look for call quality between the 3 phones.. it is a phone
If you ask me what one i would get, i would say the Galaxy s2 seeing that i have the galaxy s already
But seeing that you are new to the smartphone? The screen size will be a huge factor in choosing, see which one you like.
All 3 are great phones, there is no wrong answer. They will all last until your upgrade