The updates from 09 to 11 were mainly cosmetic I think. The 11 has the red B profile tubing in the frame while the 09 used the black rectangular section tubing. Plastic and detalj are also different. As far as I know, the frame geometry didn't change. If there are differences engine wise, they are not significant. Perhaps the biggest difference is what type of suspension the bike has, but that depends on more than the model year.
Take a lok at the Gasgas website, it has both pictures and data of previous models

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Of course you want an 18. Why buy a 17 at this point? They have been same bike for years. You could save money and buy a used 15 and it would be same bike...lol.

Yeah it is heavy...get the 18 no doubt.

I agree.. however the 17 is the refinement that has taken place since 2012. The 18 will be the start of something new, which could include some minor tweaks to chassis/geometry in 19+ ..however 19 is rumored to have a new engine as well. 2020 new bike :D