What are the differences in these boat?

Length, width, storage, max HP.

To follow up on what Speedy said look here http://www.bassboatcentral.com/rangerspecs3.htm, it gives you more of the specs

I've fished a 681 for 10 years. The 681's a superb fishing boat for its size on the lakes I fish regularly (Red Cedar Lake @ 1841 acres, Delavan @ 2,000 acres, and Ouachita @ 35,000 acres.) Don't need a full size truck to tow it. My Astrovan does very nicely. Not sure I'd want to try a 690 with the same tow vehicle but you might already have a bigger tow vehicle.

What do you want your boat to do for you? The 690's the bigger, heavier boat. Inherently it'll do some things better than a 681. I don't need those things. You might.

The 681's a great size for just two. You can fish 3 guys in a 681 but it's tight. Don't know how the 690 would handle 3 but I'd have to bet it would do so better than a 681.

How do you think the 690's would perform in shallow water like 2' or less trying to get into the back waters of the Mississippi River for Bass Fishing when desired