Get on the cat website and "compare" models and you will know what the difference is.
ERA is right for once, and get the longer track version since it is wasted to put all that power down to a track that is any shorter than that...unless you simply like to do spin-outs all the time. You might do well to read the reviews on this sled prior to a purchase.
I own the ext and my daughter rides this sled, and loves how easy it steers compared to the polaris FST. They did a nice job on the steering geometry and who needs power steering??
And you can start almost any 4 stroke with your dress belt, as they start quite easily, PROVIDED that you have battery power. Like ALL 4 stroke sleds, with efi, you need full battery power or they won't even fire....starter or no starter. If you are worried about it, throw a light set of jumper cables in the machine just to cover this unlikely event.
Don't knock a 4 stroke til you have put some time on one. They are heavy but they are lots of fun and you don't smell like an oil can at the end of the day.