I was recently making the same decisions as you, except I already had the Adventure. The best bet by far is to buy the AI from the start, although the initial $ is more, adding the conversion kit later is $$, and you can start enjoying the kayak fully right away!
If you went with the conversion kit, you will not get the 'newer rudder upgrade', it is available from Hobie at extra cost, but you would also need to install the larger gudgeon, also available from Hobie. These can only be used on the '11 and newer hulls. The conversion kit has the V1 aka bars. I'm not sure about the warranty on the conversion kit. imo, It doesn't look too difficult to install, other than making sure you're careful cutting the larger mast hole.
The '11 AI will include the 'newer rudder', very noticeable response improvement compared to the large sailing rudder, V2 aka bars, which I found very easy to assemble compared to the V1 aka bars, ST Turbo fins which are necessary, plus you'd know everything is rigged and setup properly and warrantied.
I just p/up a 'new' left over '11 AI, sold my Adventure, and couldn't be happier! Took it out today, brisk wind 15+knts, very exciting ride! The authority of the 'rudder upgrade' is great! I'm curious to see and feel the difference using this rudder compared to the way the large sailing rudder worked while in non-sailing mode. I'm sure the turning radius and control will be much improved.
Good luck with your decision. The folks posting on these forums are a wealth of knowledge that helped me in my decision. BTW, if I did not sell my Adventure, I would have done the conversion and installed the rudder upgrade. In hind site, I wish I bought the AI from the start!
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