AT&T has a "soft" 5GB per month cap, so yes your iPhone data plan is better than Hughes'. Though AT&T 3G is only available pretty much where wireline broadband is plentiful, and even then most places don't have it.
Oh, and do yourself a favor; put your SIM in an HSPA card if you want data speeds. The iPhone is HSDPA only so latency is 2-3x HSPA and uploads are a good bit less as well. Thugh HSDPA > HughesNet.