The big difference between an iPhone and the MiFi - or any other AT&T data device, for that matter - is a protocol called HSUPA. In a nutshell, HSUPA means fast uplink bandwidth. It's counterpart is HSDPA, which gives fast downloads. The iPhone has HSDPA, but lacks HSUPA. If I perform a speed test with the iPhone over 3G, then connect the iPhone up to the MiFi and repeat the exact same test, the result is that the download speed is the same for both. But in the upload direction, the difference is marked. Usually the MiFi can get an upload bandwidth of about 1-2 MB/sec, but the best I've been able to see on the iPhone is about 256 kB/sec.
Interestingly, when operating over the microcell, the uplink bandwidth is further constricted - you can only get about 64 kB/sec. That's a small enough uplink channel that it starts impinging on the acknowledgements on the downlink side, limiting how much bandwidth you can really get in that direction. But, of course, if you're near the microcell, you're within WiFi coverage anyway.
I've been taking my laptop to work and using the MiFi while on the train. It works well, but there is one AT&T dead spot between the San Antonio and California Ave stations that is troublesome. I don't particularly take this, as many would, as evidence of systemic deficiency in AT&T's network. In my view, all of the cell networks suck. They all have dead spots and none of them are in a giant hurry to fix them, it seems. But every time the phone or data card drops out, I faithfully use the "Mark the Spot" app to tell them. For what that's worth.