Balley, nice to see you back !
True, most all Shingles really look bad after 15 or so years, and the Warranty usually just covers leakage, for the most part.
My observations are, that when I actually put a 20yr beside a 30, i could see some difference in the thickness of the Material, and maybe more "tar" (asphalt).
I'd say to actually go to a Dealer, make him open a pack or two of ones you are interested in, and just take a look at um. compare !
Also, I've seen Shingles that were only guaranteed for 20 years, that were still holding up very well, well beyond that, not that they looked all that good. It's possible too, that the "Color" may be more permanent (sturdy), with a longer guarantee. I've just not used the type you're are referring to.
But, just in case there is a problem with them, if the difference in cost is not that great, maybe a 30+ would be a little insurance?
My best advice is, go look, feel, set a cheep one besides a more costly one, you'll see the difference.
Have a good Day !
S.W.
PS: A 2100 SqFt house is not a "Bungalow", it's a "BIGalow" !