I had iphone 4 and samsung galaxy s2....really enjoyed the iphone 4 alot more. more batteries, the apps just run a lot more efficiently. even though galaxy has winamp, ipod in iphone 4 beats it.
In short, SGS2 has Exyno CPU/GPU, faster than TI OMAP CPU/GPU. Screen on SGS is Super AMOLED Plus w/ Gorilla glass; SGS2 has HD Super AMOLED, which is a pentile screen. SGS2 has SDCard slot, readable as FAT32 in Windows. Nexus is EXT4, which you must rely on MTP for transfer.