Your opening a real can of worms here Striker. Dobs (newt) V SCT's V Refractors. ITs been going on for years
Basically Newts (Dobs named because of their mounting) are generally "faster" than SCT's (Lower f/number, good for DSOs and low light level photography, but can be notorious for coma), but its generally (I can hear the screaming now) that SCTs give better contrast on planets, yet still give decent views on DSOs (Higher f/number. They have a corrector plate that "helps" correct coma). Having said that Apo refractors give the best views on planet especially for photography, (don't hear any screaming over that one
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Now the LX55 and LX 75. I'd suggest checking out the Meade site for detailed info,
http://www.meade.com/catalog/index.html
but in short:
Refractors, Newts, Schmidt-newts and SCT's mounted on a meade equatorial mount.
The difference in the numbers? The LX75 are the latest models.
Now your wondering about the shorter tube. Because of the triple folded light path in an SCT, the focal length is about 3X the length of the telescope tube (OTA - Optical Tube Assembly).
Basically the telescope you choose depends on the use you're going to put it to.
This page helped me alot when trying to decide what I wanted
http://www.allaboutastro.com./articles.html
Good luck
Paul