I've gone back and forth on all of this and have a bit to say about it, here goes:
1. The ipad comes in wifi only or cellular+gps. If you get the cellular model you don't need to hook it up to a cellular provider to use the GPS, but the cellular model is $150 more.
2. If you get a wifi model you can use the hotspot on your phone to get internet if you need it, assuming your phone plan has that option.
3. The GPS in the cellular model is fine, but the external GPS units are more accurate and tend to lock faster. This isn't a huge deal.
4. Most people that use an iPad also connect it to an external device to get ads-b in data. The cheapest ones (Sentry or stratux) will add ads-b and GPS for a pretty reasonable price. If you go down this route you don't need the cellular version of the ipad.
5. What external ads-b device you get depends on which app you want to use. If you use garmin pilot you will need their ads-b box as it works over bluetooth instead of wifi. If you use foreflight, their Sentry box works well.
6. I find that cellular service doesn't work at all at 10k feet, so the weather data is either gathered while still on the ground, or you want an ads-b receiver that has FIS-B and can get weather data from ads-b ground stations.
7. If you get a higher end ads-b receiver they come with a backup source of attitude. It's not a great source of attitude, but when combined with an ipad app that supports synthetic vision, it might give you some helpful data if you find yourself in FU or IMC.
8. The size is personal preference, but I don't like having a huge ipad in the cockpit and the mini isn't really any smaller than anything on the panel, so mini for me.
9. Ipads that have direct sunlight on them heat up, and when they do they dim and get slow or quit. If you are flying on sunny days, you must blast some air on them, or take them out of the case, or shade them, or all of the above.
10. Ipads are nowhere near as bright as little garmin handhelds. Sometimes direct sunlight visibility is an issue.
11. The cables and cords get in the way if you are charging while flying. A GPS enabled model using the internal GPS will go through batteries much faster than a wifi model.
12. One great advantage to aviation apps on an ipad is that they are always updated and always have the latest charts, etc. A disadvantage is the yearly cost.
All of this adds up to mean:
I would take a wifi ipad with a Sentry Mini over a cellular ipad while in flight because I'll get weather and traffic, and it's not much more cost.
I still like the garmin portables, especially when paired with their ads-b in receiver. You still get maps, sectionals (though not constantly updated, unless you pay a bit to keep them updated), TFRs and weather through FIS-b, as well as a box that's not prone to overheat or dim. In the air I think they are great, but they won't help much with the flight planning.
Hope that helps,
schu