You aren't really going to gain much out of just an exhaust. The difference between Magnaflow and Flowmaster will be very minimal, especially on a 4.7. I'd honestly go with whatever you think sounds better.
Magnaflow will flow slightly better because of it's straight through design while the Flowmaster will be louder because of it's chambered design. IMO the 50 Series HD muffler is a great choice. Nice and loud when you get on it, but no highway drone at cruising speed.
Swapping out the Y-pipe is also a great idea since the stock one is very restrictive. IMO just swap that out and then get a cat back dual exhaust system. I had the Flowmaster American Thunder system on my 4.7 and it sounded great (came with a 50 Series HD muffler). There's a sound clip linked in my signature below if you're interested.