The answer to your original question is simple - BOTH
Late hopping is great. I've been doing the majority of my beers with only 30 minute or later additions, and I love it. There's something to be said for cost savings if you're only trying to add bitterness and not flavor/aroma, but for a few extra dollars, with the right hops, hop-bursting can produce a ridiculously good beer for many different styles (not just IPAs).
But seriously, back to your question. The real answer is of course complex. It all depends on what flavor and aroma you're trying to get out of that 20 minute addition. I assume you're saying 1 oz. of Simcoe vs. 3 oz of Cascade to provide similar IBU, but those additions will be quite different with respect to flavor, aroma, and the character of the bitterness. Simcoe is much more piney or 'dank' whereas Cascade is more citrus, so you're going to get a much different flavor/aroma.
With respect to the character of the bitterness, the cohumulone levels of the hops affect how aggressive or assertive the bitterness will be, so for a given IBU level, a hop with a higher cohumulone level, expressed as a percentage of alpha acids, will produce a more assertive bitterness that can be perceived as more harsh. That's why some of the new purposefully bred high-alpha aroma hops are also touted as superb all-purpose hops, b/c they have low cohumulone levels. Simcoe is ~15 - 20% and Cascade is ~33 - 40%, so a given IBU level attained through a single Cascade addition may be perceived as 'harsher' than the same IBU level attained through a single Simcoe addition.
20 minute additions are probably late enough that you won't get too harsh a character from the Cascade addition, but it is something to think about. I always try to keep my 'heavy' late additions to high-aroma, low cohumulone level hops to get tons of flavor and aroma and smooth bitterness.
Good luck.