From the current observation alone, the iPAd WiFi + MiFi is king, compared to the standalone 3G.
Pros and Cons of the two different version, all IMO of course:-
iPad WiFi + MiFi+ Battery saver! You have steady power drain because of steady connection between your iPad and the MiFi. Power depletion is much steadier, which gives that peace of mind that you won't run out of power in your daily commute/routine.
+ Depending on the specs of your MiFi, you can now get reasonably accurate position compared to WiFi only position triangulation. This opens up the possibility of using the Wifi-only iPad to navigation.
+ Since the MiFi is with you all the time now, for road warriors using other devices that can tap onto WiFi connection, this provides another option to go online.
+ Aesthethically more pleasing than the 3G, with a clean aluminum back, compared to the black-strip on the 3G model
+ You can reuse your current SIM card on the MiFi without the need to mutilate it further to fit into the 3G SIM tray. The annoyance of having to swap between your phone and the MiFi may be a deterrent to some to do this often. Also, you avoid needing to subscribe to another data package to be use solely for the MiFi.
- Apps that require GPS positioning, will not be able to take advantage of the WiFi model. This is for future-proofing purposes, for example an app that requires pinpoint accuracy to work, such as location-aware apps, etc.
- You're carrying two devices instead of all-in-one.
- Once your MiFi battery power depletes, you're stuck with searching for WiFi hotspots for your connection. Most MiFi solutions last 5 hours *advertised*, and unless you want to bring its charger (add another device), you're stuck without a connection until it is recharged, or you're in another hotspot.
iPad 3G+ All-in-one! The constraints of moving around with finite amount of space that can be brought with you in this case is a huge plus. Travelling light with the iPad is the whole idea here.
+ Built-in, GPS component with A-GPS. This gives pinpoint accurate positioning if you're concerned with apps that uses navigational properties.
+ Future-proofing, to a certain extent. Granted, there may be future updates to the iPad, but with every feature sets on the device, all available apps, present or future will be able to take advantage of the device fully. May be obsoleted by future add ons to the iPads like cameras, etc.
- Battery hog! Moving around with the device alone has shown that power draw is significantly larger because of the need to remain connected, the device will compensate by increasing power consumption to catch the 3G signal in low-strength coverage areas.
- Aesthetically less pleasing in looks because of the black strip
- Unless you're subscribing to service operators with support for microSIMs, mutilation to fit into the 3G iPad is necessary. May entice users to subscribe to a secondary data-package only service, which adds to the already burgeoning cost.
Bear in mind, these are my pros and cons, these points may or may not be important to you, depending on how and where you use your iPad. Feel free to add your own take on this.
The best combo I'd see would be the 3G iPad + MiFi, for that maximum connectivity + battery conservation purposes. You can turn off the 3G wireless connection while using MiFi, and once MiFi battery depletes, you can either already be at a hotspot, or use the onboard 3G connectivity.