Rasky definitely covered it well.
I think you could get an LHR15ES and be perfectly happy. Its a great tool that features impeccable balance and will be a great solution, especially if its just for your personal use and you're not correcting paint as a career. When you spend the extra $100 and go with the Mark II you get closer to 40% increased power (more than the 21ES vs 21II) and this makes the Mark II a monster! As pointed out above you'll run the Mark II tool at 3-4 on the dial, only really ever jumping to 5 or 6 when you need to maintain pad rotation on complex curves. The torque delivery of the Mark II, considering the fact that it still only consumes 500watts, is very impressive.
So long story short - you can go with either and probably be happy, but if you're a tool geek or intend to do more than just your own car as a hobbyist the Mark II is probably the better investment.
For the long cord: Thats an offer exclusive to Autogeek and its worth it if you find yourself struggling for the right cord setup in your garage. For me personally my nearest outlets at home are not convenient so I end up using a 50ft extension cord regardless so the longer cord doesn't make much difference to me there, but if you have well placed outlets the ability to plug right into the wall and go is nice, especially for the small amount of additional money.