As kc2ine noted, the X1 interface is the same, regardless of the version, so learning your way around X1LE won't be wasted time.
Considering you are working with your MIDI keyboard, there is something to be gained by going with any of the X1 non-LE versions. You get soft synths.
TTS1 is a decent, basic GM2 synth, which is nice to have around.
More synths equals more possibilities.
Here is a comparison chart of X1 Producer, studio, and Essentials: http://www.cakewalk.com/products/SONAR/SONAR%20X1-Comp-Chart.pdf
Consider your needs.
For myself, X1 Essentials was a good fit, especially with my current hardware setup(XP, 32 bit system).
One very big thing to consider is that X1 is still evolving. Updates will apply to X1 P/S/E, but not, apparently, to LE.
This has been documented elsewhere, but I am open to correction.
Here are the X1LE specs that I can find: http://www.cakewalk.com/Products/SONARLE/feature.aspx/About-SONAR-LE
It would be fairly easy to max this out, even with a fairly modest project, so even X1 E would be an improvement.
If you haven't already done so, a decent interface/ sound card is a must, especially if you will be recording with your MIDI keyboard realtime.