Anything but S7 is a waste of time. Especially having in mind expected mining diff increase. If I were your, I do go PoS or build a rig cause SHA 256 mining for home users is the thing of the past. Unless you are a whale.
Definitely not a whale. 
What's PoS? And building a right, you mean, use GPU mining?pos is for altcoin, does not belong here, it's proof of stake
anyway it's not true that only the s7 has a reason, s5 is good if you don't mind electricity, because it cost much less and the roi is faster
s5 deliver 1/4.2 of the hash, while costing 1/6-7