I've used 3G direct on the iPad and with wifi to a mifi, for me the iPad 3G wins hands-down on convenience.
- Permanently online, no waiting 30 seconds before you get Internet access
- 24x7 push email and notifications
- No need to keep a second device charged
- I have an AT&T microsim which I use when Stateside ($25 for 2GB is far cheaper than roaming)
There are benefits to a mifi, such as being able to share the connection with multiple devices, but I just love the instant on with instant access.
I grabbed a free T-Mobile PAYG SIM, topped-up £20, text 6MONTHWEB to 441 then cut down the SIM and inserted into the iPad. That's me online for 6 months.