if you have to give up unlimited data then DO NOT switch. Hspa+ on att is pretty fast and vzw's shared data plans are a complete ripoff. What's the point of LTE if you use up the data in one day?
I had unlimited with AT&T, but AT&T only has edge in my area, so it is terrible (moved from an area that has HSPA+). Verizon has 3G and has announced LTE coming within a few months. No plans from AT&T for HSPA+, so hence my choice to switch. Edge was just painful, so I would prefer to pay 1/3 more for decent data speeds.