I am a Motorola boy here but would jump all over the Samsung g3 in this case....much larger screen and has ice cream sandwich...a much higher end OS.
4 inch verses 4.8 inches is major as the square rule applies in relation to square inches..thus the screen is actually 44 percent larger..this is major especially when you go to hit those little touch keys while the phone is held straight up and down.
You will be able to do it on the larger one but not the smaller one...also browsing will be much more enjoyable as you wont have to zoom as much thus get to see a much larger area of the actual webpage as well.
Also you will able to hit those small text links easier while zoomed in the same amount as the smaller phone.
However it is a delicate phone compared to smaller phones...when a phone falls and hits it can either cause a torsional twist,breaking the glass or simply impact the corner and break..
Many people like the preloaded software and touchwiz interface of the samsungs in general..they are a little more user friendly...just something to consider.
However the android market is huge and you can fix up that motorola to make it friendlier.
I had gotten a samsung recently but took it back 6 days later...reason being it didnt wouldnt hold a tower that great.
I had to use my passengers Motorola to make a call about 90 miles from home....so the samsung had to go,it had a dead zone for about 5 miles of road while the Motorola never lost service.
The motorola I got has a slightly brighter screen,better sound,better service,better feel in my hand, a faster charger,brighter flash although I did like the camera better on the samsung...It was a mesmerize which is a galaxy s1 i believe,now the s2 is highly praised in general and has a pretty good track record.
I have the Motorola electrify..which is a Motorola photon...nice phone I do wish it was larger though but thats the largest my carrier had in Motorola--- 4.3 inch screen.
On to cases...a necessary evil if you drop a 700 dollar phone..and you will drop it..they are designed as such...those smooth beveled edges are not for looks.

They are easier to use with a case as well as you have a little rubber hitting your hand while using the keyboard when the phone is sideways....can be easier to dig out or your pocket due to the cutout for the lens and flash hook a finger in and pull.