I would make the switch even at 70Mb a day your still under their radar ive been well over that several times on the att sim. The iphone 3G or 3GS both are decent phones for basic use even Facebook and light web browsing. However you will not have access to HSPA+ on those phones and will need to get an att sim for 3G speeds. As for phones the new Nexus 4 will get you HSPA+ on both att and Tmobile sims a used iPhone 4S will get HSPA+ on att and may fall into your price range as well. Good luck in your choice.