Great question, fun topic... although probably not a new thread as I've seen this question before and I've always resisted answering....

I can resist no more.

Both downriggers will serve you well. Each are durable, very dependable and as long as you're choosing one of those two brands you really can't make a bad choice.
Each make has it's own advantages and as I've used each extensively, allow me to offer the following:

$cotty's will co$t a little less up front but like anything else, you get what you pay for. If you are a weekend warrior, using your 'riggers a few times a month and are on a budget then they might be the right choice.

Cannons cost a bit more initially but they do more. Positive Ion control is Cannon's "Black Box" and is built into each and every one.
The fastest rigger on the market? I've had both brands bolted to the boat, put identical balls on each and my son and I hit the buttons on the count of three. 150 foot pull with 12lb balls, Cannon was the winner by 9.5 seconds.
If you want more exotic functions, only Cannon will bottom-track, jig and interface with your sounder.

It all depends what you want and need in a rigger. Chances are you will be happy with either brand but for me the choice is Cannon.