I have the 20/15/15 and I recently ordered a 3rd 15. The 20 is a bit too big IMO but I'm going to keep it for batches that require over 28 pounds of crushed grain. That's the limit a 15 will take IMO. Go all 15's, you'll be fine.
On Tap: Belgian Pale Ale, Session Porter, American Brown Ale, Rum Barrel-Aged Wee Heavy, Beer Camp #118 Grandma's Closet
On Deck: Oud Bruin, Blue Corn Saison, Weizenbock, Berliner Weisse, Pomegranate Cider