I am currently galaxy s3 user and it is a great phone with 1 big drawback. Its ram is not enough. 1 GB is not enough. Because of that i was waiting galaxy s5 as 3 gb ram. but stupid samsung made it 2 gb ram. now i am not planning galaxy s5. perhaps sony xperia z2 but prices are really important factor. what do you think ?
If you upgrade your S3 to Android 4.4 you will probably find 1gb can do a lot more. Android 4.4 greatly reduces ram usage leaving a lot more free for your apps. My Galaxy Nexus also has 1gb of ram and feels like a new phone after upgrading to 4.4, i honestly think 2gb is more than enough. I'm personally not a fan of the Galaxy S phones. If price is important then get a Nexus 5, i personally think this is the nicest Android phone out at the moment and is a very good price compared to other high end devices.
My HTC One has 2GB RAM and i'm still on 4.3 and I have no issues multitasking. Honestly, 3GB of RAM is overkill and not really needed.