Steem's price is pegged to three main factors. 

1. The demand for Steem in the open market.
2. The currencies it is paired with in major exchanges.
3. Price of Bitcoin and Ethereum and other cryptos traded in fiat pairs.

So, the answer is both of the reasons you mentioned affect Steem price and some more factors are involved in it. 

There isn't much to explain about buy sell demand since it is general knowledge. So, let me skip to reason two.

Steem is not traded with fiat pairs. It is mostly traded with Bitcoin pairs in exchanges. So, when ever Bitcoin price goes down, even if Steem price doesn't go down in BTC value, it goes down in $ value because of Bitcoin price going down.

Both

However a strong buy and sell volume can trigger a faster rise in Steem price.

Imagine, for instance, 1 million dollars was used to buy Steem this moment, the price of Steem would rise exponentially regardless of what's happening to bitcoin.

The decline / increase that occurs in Bitcoin will indirectly affect the interests of all Altcoins.

Bitcoin has a market dominance of more than 40%, so we can see that this is a reflection of all other crypto currencies.

So, when the price of Bitcoin falls, interest in altcoin will automatically drop because many people think that altcoin will also decline. FOMO always makes the market fluctuate significantly.

The Steem price unfortunately is pegged closely with whatever Bitcoin does. It is fantastic when Bitcoin rises but less so when it drops.

When purchasing Steem on the exchanges we normally have to go through Bitcoin or Etherium first which has  a huge impact as it is traded in pairs.

The demand for Steem on the exchange also has  an impact on what the price is going to be. The less Steem that makes it to the exchange the better as the scarcity factor would increase it's price. This is why SMt's will be good as they will hold a lot of the Steem being tied up. If an investment company purchased a huge amount of Steem as an investment the  price would rise regardless of what bitcoin is doing.

We are hoping in the future that Steem will be big enough not to be paired with Bitcoin and you could purchase with money directly.

Price is affected the the both Bitcoin and the demand and supply.

All the Alt coins follows Bitcoin because:

Bitcoin is the founder currency and holds the majority shares in the crypto world. Market dominance of bitcoin is around 32-35% of total crypto currency volume. 

There are about 2500 plus crypto currencies and no other coin has such a huge dominance. Therefore, all the crypto market follows the Bitcoin. 

Secondly, price also dependent on demand and supply rule.  If demand is increase because of certain news or announcement the price will go up and vise versa.   

Both.......

The entire  crypto ecosystem is still in a stage where the market sentiment is largely driven by the sentiment of bitcoin and if you see the recent breakdown of crypto market including steem, it is largely a reflection what happened around bitcoin.

Although the buy/sell and the supply/demand of a particular currency always count on its own but its sentiment is driven by either fundamental factors or the broader market sentiment around the currency like bitcoin.

The next couple of sessions for bitcoin is going to be interesting and also for steem.

Thank you and Have a great day.

Bitcoin is the first crypto ever created...

And the rise and dip of bitcoin's price effects the whole market, at least psychologically.

So yes BTC impacts steem price like it affects the price of other alt coins...Bitcoin controls the crypto market sentiment!!!!

But there are instances when there is slight selling pressure on bitcoin, because investors are pulling their money out of BTC and investing in alts...