Ok lets see:

1.- Purchase $1000 worth of steam and hold it for 2 years

You have to keep in mind that "steem" has a high inflationary rate, so here you will be betting that the growth of the platform will be higher than the actual inflation.

2.- Purchase $1000 worth of steam power and hold for 2 years.

If you put your money on steem power you gain interests, you can also upvote content (which doesnt require much from you) and gain some curation rewards. I think this would be your best option.

3.- Purchase $1000 worth of steam dollar and hold for 2 years.

Steem Dollars will always be worth the same amount of real USD, YOU WILL GET 10% annual reward, so in 2 years you would be looking at 200 bucks. If you want to specualte this would be the worse option.

Good Luck and let us know what you decide on a follow up post!