If you expect to get weather in the air, you'll need one of the ADS-B-in gizmos in addition to an iGizmo. If you're running Foreflight, the only ones that work with it is the Stratus and Stratus II. I have the Stratus II and an iPad Mini with both wi-fi and cell, but in the air I shut off the cell coverage. If you're running a different app than Foreflight, there are other ADS-B-in gizmos to consider, all pricey.
On the ground, if I'm not near a wi-fi hot spot, I'll turn on the cell coverage, and I can then get anything I could get through a home Internet connection. However, unless I'm in a 4G LTE range, the downloads will be noticeably slower. If you were to try to download weather in the air using a cell connection, you'd be mightily disappointed, I think, as coverage would be spotty at best.
This is the ADS-B coverage map as of mid-November:
http://www.appareo.com/wp-content/uploa ... 75x297.pngCary