Bitcoin to dollar is more stable than bitcoin to altcoin. Dollar can't turn scam while altcoins can. On top of that, an altcoin values can rise or fall pretty quickly. So I prefer to trade from bitcoin to dollar.
Yup Dollar is really better I guess. Most of buyers and sellers are USD to BTC only in many sites, Some sites like Yobit are useful to sell bitcoins for all kind of coins and fiat as well. I personally suggest you start trading in Localbitcoins.com and Kraken are good in BTC trading normally.