Metaio also supports cross platform development using AREL in HTML5 based apps
See: https://dev.metaio.com/arel/overview/
Metaio also supports Edge Based Tracking which is very robust and does not suffer from lighting or texture condition changes.
For simple Location Based AR using GPS etc. This can be achieved without the need for any 3rd Party AR SDK.
Apple have free source code for this here:https://developer.apple.com/library/ios/samplecode/park/Introduction/Intro.html
Vuforia have a very useful feature called extended tracking which allows tracking of the environment even when the original marker is no longer in view. This is a great feature and worth investigating.
As for Layar I was not convinced