everyone should decide themselves I was just giving this option, as OP didnt seem to be very well informed. The chance of hitting a block with an S9 is indeed very slim.
I think it would be wiser to advise him not to mine with an S9 in the first place, going solo with an old gear as opposed to mining to a pool doesn't change the power bill, it just makes things even worse, you would be faced with a power bill and all you have on the other hand is "hope", I also think the phrase "very slim" doesn't fully describe the chances of a single S9 hitting a block, he has 1 in 143,679 chance per day, one single change in nearly quarter a million at a cost of nearly 30kWh which is a huge bill, he would have a lot more chances winning the lottery while spending a lot less.
So I agree with NFW here, solo mining is only 'ok' if you use a small USB stick that needs a few dollars worth of power a day, anything aside from that is just irrational.
OP, it's good that hzti brought the power cost in this topic, I hope you understand what you are doing and are well aware of the cost you are going to pay vs the rewards, if you already ran the numbers and decided to go for it, you must focus mainly on the minimum payout of the pool, try to stick to a PPS pool which has a very low payout threshold so you don't have to wait for months to get paid.
Binance, Viabtc, F2pool, and the list goes on and on, you can do a google search on all PPS pools and then read their terms to find out which one has the lowest payout threshold, even at a bit higher fees it would still make sense to use that pool as opposed to saving 1% on fees only to get paid out in 4 months.
