Plexiglas is much more prown to cracking. Lexan is much more difficult to crack. While Lexan has some advantages -- it's clearly not as clear.

Here's a very good comparison: http://www.plasticgenius.com/2011/08/plexiglass-vs-lexan-matchup.html

Plexiglass vs Lexan Matchup
I don't know how many times we have been asked the question - What's better: Plexiglass or Lexan?

Depends. What's the application? Let's match up some KEY properties to see:

Plexiglass (acrylic sheet) aka Lucite, Plexiglas, Perspex, Acrylite

92% light transmission - clearest plastic sheet available - about 15% cheaper than polycarbonate
large variety of colors available
light pipes through the sheet edges
does not yellow or discolor in the sun - 10 year warranty regarding 2% loss max.
optically water clear in all thicknesses
inert - no outgassing
thermoplastic - working temp of 180F continuous
burns very nicely after you remove a flame source - fire rating UL94HB
scratches - can be restored by sanding and buffing
resistant to breaking - when breaks pieces are large and dull edged
recyclable
easily bonds with solvent adhesives to itself and other plastics
hydroscopic (will absorb a small amount of moisture on side facing moisture (warps)
available up to 4" thick as a monolithic cast sheet, and thicker via custom casting

Lexan (polycarbonate sheet) aka Tuffak, Makrolon

88% light transmission - very clear - about 15% more expensive than Plexiglas acrylic sheet
limited colors available (white, black, bronze, gray, sign colors)
light does not pipe through sheet edges
untreated sheet discolors in 5-7 years of sun exposure - UV coatings available to extend 15 years
inert
optically clear - loss of some light transmission in higher thicknesses (5% in 1/2")
thermoplastic - working temp of 275F continuous
self-extinguishing fire rated 94V0 - UL rating depending on thickness
scratches - cannot be restored by sanding and buffing
resistant to breaking (virtually impossible to break)
recyclable
easily bonds with solvent adhesives to itself and other plastics
bullet resistant grades are available - all grades of BR levels
hydroscopic (will absorb a small amount of moisture on side facing moisture (warps) and therefore difficult to thermoform without pre-drying
available up to 1/2" thick as monolithic extruded sheet, up to 2" as a molded sheet

Applications:

Plexiglas Acrylic Sheet:
Safety glazing (up to 17 times stronger than glass)
Boat windows and hatches
Machine Guards and Electrical covers
Display cases
Brochure holders
Picture Frames
Lighting
Anywhere you need a clear, impact resistant, easy to cut and maintain sheet

Polycarbonate Sheet:
Safety glazing (unbreakable)
Bus Windows
Machine Guards and Viewing ports
Anywhere you need a clear, unbreakable, high working temperature, easy to cut sheet