However even after creating a TM from the translated dummy/trial file, I am unable to use it on any other dummy/trial file. What can be wrong?
What exactly is the problem? Is an error message displayed or doesn't anything happen at all? - The only thing that I can think of without any further information is that you might be using different language pairs for the TM and the ITD file that you are trying to translate. The TM needs to use exactly the same languages (source and target) as the file that you want to translate.
What happens after the one month trial period ends, will I be able to use the demo in any way?
During the one month trial period you have a fully functional version of SDLX. After this period it will switch to demo mode. In demo mode you cannot use all functions and only translate files that contain less than 1000 words, AFAIK.
Is SDLX significantly better than Wordfast?
I cannot say much about this because I'm not too familiar with Wordfast. - I would expect that this depends a lot on the file types you work with.