They are quite similar, but the difference would be that 'slack' would probably refer to less work, or being less busy. 'Low' would refer to lower productivity, or lower profits. They would usually be the same, but not necessarily. e.g. A farm might have a low year due to low prices, even if there was a normal amount of production, and work. Or a software company might not have much work for its developers, so they would be slack, even if the profits were still OK from sales.

If you have any more questions on this please contact me directly because it doesn't show up on my profile as it's in the comments.

"Winter is the LOW season at most hotels in this seaside, because very few tourists come to stay." Summer is high season, but on account of the economic downturn the season's been slack.

Winter is the ___ season at most hotels in this seaside, because very few tourists come to stay. According to your reply, what should I choose?

I see it in terms of a holiday resort. There are popular times of the year, usually school holidays, public holidays, etc, which are "high season", when prices are much higher and holiday accommodation is usually completely booked up, and then the rest of the year with much less demand, and consequently lower prices, which is the "low season". A "slack season" would be an unsuccessful "high season", i.e. it would be expected for everything to be booked up, yet it is not.