Best 3G Options for Macbook Air?

The MacBook Air can handle it. And if you need to travel a lot to different sites then I strongly recommend the MacBook Air. As for 3G connection, not sure where you're located but reports has shown that Verizon is pretty good and AT&T has a fast download speed. I would go with Verizon first though it cost more then AT&T as the second choice.

They have something called the MiFi, it's a pocket size device that creates a Wi-Fi network from the carrier's 3G network.

Is MiFi better than simply using USB dongle for 3 G ?

Mifi is definitely better ergonomically than the USB Dongle approach. They look big and cumbersome on a regular laptop, and even more so on a MBA. I know, because I have to use one for work related activities. The limited number of USB Ports also is an issue. The dongle will not work on the Magsafe Side with Magsafe plugged in, due to physical interference. The same issue comes up on the Display Port side if I want to use my Display Port adapters. I wind up using a small extender cable.

Mifi needs no mechanical connection, as it works as a standalone WiFi Hot Spot for up to five(5) connections. MiFi devices are available from all of the major carriers, including a no-contract prepaid program from Virgin, which is backed by Sprint.

I use the Verizon MiFi "deck" with my MBA and it works great. It has a couple of important advantages for my application: it doesn't take up a valuable USB port and it broadcasts a WiFi signal that can be used by up to five devices simultaneously.

Battery life on the MiFi is good - I get about four hours of Internet usage per charge. It's small enough that it's no problem to carry in my messenger bag. I really like the ability for my wife and I to be able to use the device simultaneously.