The answer as to what app to use on iOS for GPS tracking?
GPS Kit hands down. I've been using this app for over 3 years now. They've made all kinds of good features, and the map caching is really nice.
You simply move the map to the area you want and zoom to the desired level. A yellow overlay represents the bounds of the area to be cached. If you select an area with too many tiles it will tell you. Each area you download is saved as its own set. Unfortunately, you can't name these sets. This would let you go back and delete an area if you didn't need it anymore, based on the name you specify instead of "Rectangular Area".
I should also mention you can select the provider of the map tiles, such as Bing, Google satellite, terrain, etc.
It's easy to email your tracks and waypoints, post them on Facebook or Twitter.
You can drop a weather station icon on the map to get updates in that locale.
To import a gpx file, just email it to yourself, open the message on you iOS device, click an hold on the gpx file. You'll get an option to import into GPS Kit.
There's a lot more to the app, but that's most of the juicy bits that I use.