Didn't really transition into the Pirahnas so much...I had been wearing the DS Trainer and had some foot issues due to the support. I'm not a huge guy (5'9", 150lbs) and a natural mid- or forefoot striker so I figured I'd try my racing shoes in a training run. Liked it, started off with 2-milers in them for a week, then 4-5 miles, then 8-10, and finally all my runs up to 12 miles - total transition time was like a month. Just make sure to stretch your calves pretty regularly, they'll take a beating at the beginning.
I picked up the Hyper Speeds for longer runs and now use the Piranhas for all my shorter and faster stuff. The Hyper Speeds definitely feel more supportive and bulkier than the Piranhas, but they're still nice and light and have a 5mm heel/toe drop.
Go with the Hyper Speeds, you don't regret it. And depending on what size you are, there are some pretty cheap HS2s on eBay (not mine, just saw them cruising around online).