I have experience with mining bitcoin not with the s7 or s9 though, I was mining bitcoin with the s1 and then s5 from 2014-2017 and made a very nice amount of bitcoin. I wouldn't use an s7 as the watt/gh ratio isn't good compared to the s9. The only thing is when you mine bitcoin timing is everything you want to get as much out of each machine as you can and you don't want to buy a new machine and then 1 month later the next asic comes out and the difficulty goes way higher you also have to consider your electricity. From my experience from mining since 2014 you have to also be good at projecting your roi for example you pay .45 btc for the s9 (not including psu) go to https://www.coinwarz.com/calculators/bitcoin-mining-calculator and change the difficulty current difficulty is 711697198173 so you should change it to something that wont happen in 6 months (or at least shouldn't)
EX: 1111697198173 that is a really huge increase now check are you still making money after electricity? Then from my experience you'll roi and make some extra. I personally made way more btc then I thought id make and that was with people saying mining is dead and you can't do it unless you spend a million, fact is they're wrong you can make profit with an s9 if you meet the criteria. My last bit of advice is do your research on the s9 and t9 even though the t9 uses more electricity it apparently (according to other people) is more reliable and doesn't die as fast lots of people had issues with the s9 from what i've read.