track ?? and 600 ??

ok 2 questions, ill throw them up in one post.

1. i have a 2007 dragon 700 155". the track is a series 5.0 correct? is there a noticable difference if changed it to a 5.1 series 155" track?

2. thinking about buying a 600 for my girlfriend. maybe a 2007-2011. what are the big differences between all these years if there is one? motors all the same? which would be the best year to buy? dependable, reliable, longevity, great sled?

thank you. i had someone tell me to switch to a 5.1 but didnt know why and before doing it thought i would ask here. also, the track i have is in excellent condition so i wll run it as long as i can, just didnt know if it would be worth the investment at this point to switch. i love this sled for the 200 miles i put on it since i bought it. gona do some gearing and clutching on it and that is probably it besides hot air vents. already did better boards, and it came with a slp pipe and can and intake deal.

yeah i would like to find a 09 600 shift 155 probably sometime. i chose 09 cuz the better bumper, and maybe cheaper cuz year older than 10 and if its all black she can do whatever to make it pretty for herself and to match her gear. would like to find one to let her try out tho first before i spend the money on the thing