I own a Cortes, and I find it to be a great detector, especially in parks and school yards. The display is accurate as far as junk vs. coins. I need to qualify that statement, because I hunt in Canada, and our coins are different than US coins. This has always been a frustration with the green and yellow ID machines I have owned in the past. However, the Cortes gets around that problem by the small bar graph that displays the conductivity of the target. I have learned to ID the coins by the spread of the bar graph alone. I wouldn't hesitate to recommend the Cortes to anyone. I have dug coins at 10" and 11" (measured, not guessed), it's light, and can go 35+ hours on a set of batteries. Don't forget about the life time warranty.
However....
The F70, being $30 cheaper is the obvious winner. From what I have read, it is loaded with more features than the Cortes. If it was me, and I had that budget to spend, I would look hard at the F70. I don't know that it's any deeper, and I don't know how I would get around the Canadian vs. US coin issue, but it would be worth looking into. I would want to visit a dealer however, as buying sight unseen is too risky because of quality issues. I would want to look my potential machine over closely before buying. That's my opinion, and others will no doubt strongly disagree, but that's how I feel.
However (again)....
Look at a used Cortes. They generally go for $400-$500 and don't worry about the non-transferable warranty. If the machine has no signs of abuse Tesoro will fix it for free anyway if it needs it. That's what I did. I bought mine for $525 as a demo and it had a problem with the all-metal mode. I sent it in and had it back better than new, in less than 3 weeks, and at no cost.
Since this is the Fisher forum, I'm probably going to take a lot of heat for this, but who cares. I hope it helped a bit,
Dan