I recommend that you do not focus on the HDMI version number, but rather on the specific features you are looking for. Many of the features in HDMI are options, but support for a particular HDMI version number does not mean that the latest features enabled by that version are incorporated into the product itself.
That said, here is a brief summary of the revision changes:
From HDMI 1.1 to HDMI 1.2, the major things added were:
- Support for SACD ("One Bit") audio format
- Enabled easier implementation of HDMI in PCs by 1) eliminating restrictions on the connector and RGB color space, and 2) requires displays to support future low-voltage (ex. AC-coupled) sources
From HDMI 1.2 to HDMI 1.2a, the major things added were:
- CEC protocol and testing was fully specified
- Improved cable and connector testing and verification procedures defined
- Established a certified connector list
A final note: all versions of HDMI are fully backwards compatible. This is again a reason why one should not focus on the version number when evaluate a product.