Since I'll be using in large cities I expect I'd benefit from the higher speeds.
Both ATT and Tmobile are moving away from 3G/"fake"4G to LTE. 3G will start becoming slower after every year
your iPhone is super old... Doesn't support LTE
I'd recommend getting a modern smartphone (you can find one for every price point from $50 to $200).
iPhone 4 is fine for international (you'll mostly connect to 3G overseas), but pointless for paying so much money stateside
but back on topic on international roaming - some prepaid providers allow roaming within EU , eg
http://prepaid-data-sim-card.wikia.com/wiki/Euro3