We did a similar exercise a while back... We ended up settling on iPads for now for the following reasons:
- Support. An iPad that breaks can be exchanged at any Apple Store... usually in the same day. We buy Apple Care + with our iPads which means we can exchange them for any reason (even if it's totally our fault) for $50. The support varies widely for Android tablets, but usually requires mailing the device in for service.
- Memory. The iPads have internal memory up to 128GB. This was important for us because some of our offline tile caches are> 17GB. Android tablets often rely on SD cards for storage, and many of them can't support file sizes larger than 4GB.
- Constancy. We use Collector, but also develop our own in-house apps. Any iPad that runs the current version of iOS (iPad 2 all the way up to the latest iPad Air 2) will work just fine with any app we throw at it. This is very complicated with Android; with different tablets running different versions of the operating system. Some might be upgrade-able to the latest OS, but many receive no updates after purchase.
- Ease of management. Like it or not, iOS is a pretty locked down system. From our perspective (managing large numbers of tablets) this is a good thing. We don't restrict what people can do with the iPads we send out in the field, they are free to install apps, change settings, whatever they want... but when the device comes back to us we can reset everything to factory fresh in a couple taps. We weren't interested in buying a 3rd party management service just for data collectors and with iOS we really didn't need to.
- Enterprise Apps. This might not apply in your case, but for us Apple's Enterprise App program is fantastic. We can develop apps and deploy them to our devices using an internal app store without having to go through the Apple App store approval process. This lets us quickly develop and push out updates to apps as needed on our projects.
These might not all apply in your case, and the fact is this is all changing really quickly. We are starting to see more Android tablets with internal memory instead of SD card expansion, and Google is doing what it can to address the OS update issues. For now, for us, iPads in Lifeproof cases are where it's at for field work.
As far as the required memory - this will depend on what and how you are collecting. We do offline collection and tend to have lots of attached photos, so we are regularly dealing with large TPK files and large .geodatabase files on the devices. It isn't uncommon to have a TPK file in the 3-6GB range, and we frequently have multiple versions for a project (topo, imagery, streetmap, etc...). Photo attachments are an issue as well, with full size pictures from a tablet camera coming in at 5-8MB each. Multiply that times 20-50 pictures per day on some of our projects and the database can get really large fast.
