if you Airave does not work, there's tech support dept. just for airaves, regular tech support does not know how to work with it. as for coverage, call customer service and if they do not know ask to be transferred to tech support. they can tell you if there's an outage in the area, if it's the network vision (replacing towers) or if they need to submit a network ticket called CTMS. the good thing if it happens to be the network vision, is that once they finish the service will be way lot better and in your case would be about 20 times better. if there's an outage be sure engineering should be working on it. if they need to create a network ticket it usually takes about 5 days to be resolved.

as you know the iphone is only 3g, if I were you I would not consider that phone whithout having to metion all limitations it has because is from apple, so, stick to andorid. between EVO LTE and S3 wait to have your issue resolved first before upgrading. going to a contract with poor service is just not fair. if they resolve your issue, go for the S3.
if any other doubt or concer don't hesitate.... I know a lot about Sprint!