People were also stating there is a difference with the T version if you do try and use 3D with it. It had something to do with the monitor not supporting a certain Nvidia format. I don't know a lot about 3D, but people that purchased the T monitor were very angry because it doesn't work with the glasses they already had...
I guess in the monitors manual it clearly states this but the website doesn't have any information on it. You may have to do some google searches... sorry to be vague.
Anyway if you aren't planning to use 3D get the T not TX.
Edit: Found the difference on Nvidia's website: http://www.nvidia.com/object/3d-vision-system-requirements.html
The 2420T does NOT have 3D Vision HDMI Support while the TX does.