Each implementation have its advantages, GNode have less memory usage but suffer on speed, while TNode require much more memory but is really fast. All measurements were done using Serialization/De-serialization tests for large XML files.
GXml 0.12 provides DOM4 specification implementation as main feature. This allows to deprecated the old DOM1 one. DOM4 requires API changes, but almost all Vala should compile with little modifications.
DOM4 implementation is still incomplete, but allows to you access any XML like trees, like HTML, using a GObject API.
DOM4 was possible thanks to GObject+Vala, because most of its interfaces was really easy to port using Vala syntax and GInterface. GNode/GDocument is the only classes implementing DOM4 interfaces, but other will come.
With these lasts changes, GXml API is becoming more and more stable, suitable to try new use cases.
Some one talk about to support XPath, me about to support XSD and large XML files. May you want to leave comments on what should be the next step on GXml development. Search at G+ Vala channel for a poll.