The Internet is your friend:
http://www.roku.com/products/compare
Main differences between Roku 3 ($100) and Roku 2 ($80):
1) Roku 3 won't do composite for non-HDTVs, it only works with a HDTV. Roku 2 will work with any TV.
2) Roku 3's CPU is 5x faster than the one in the Roku 2.
3) Roku 3 can do YouTube at this time, as well as updated Netflix. YouTube client is coming in the future for the Roku 2.
4) Roku comes with an Ethernet port, a USB port and a microSD card slot. MicroSD slot is used for additional device storage for channels and apps, USB port is for accessing content via USB. Ethernet is obvious. Roku 2 is lacking those items.
5) Roku 3 comes with the motion remote, Roku 2 doesn't.
6) Roku 2 has channel shortcut buttons (Netflix, etc.) on its remote, Roku 3's remote doesn't.
Main difference between Roku 2 ($80) and Roku 1 ($50, down from $60):
1) Roku 2 has the headset remote, Roku 1 does not.
2) Roku 2 has dual-band wireless (2.4Ghz + 5Ghz), Roku 1 only is single band (2.4Ghz).
If you only have 2.4Ghz wireless and don't need the headset remote, Roku 1 works just as good as the Roku 2 and you could save the $20 difference.
Roku 3 does have a lot of additional features over the Roku 2, and for me, I went with the Roku 3 as more future-proofing for long-term use. I did get Best Buy to match Sam's Club on a Roku 3 price of $67.71, which was the cheaper price between my local stores, with other Sam's Clubs stores having the Roku 3 at $75.
If you have a Sam's Club local to you, you might want to check one of those, as the Roku 3 bundle at Sam's Club comes with a HDMI cable as well as a 1 month subscription to Hulu Plus.