Bitcoin is the base currency of crypto market and there is no logic to convert your bitcoin to ethereum as all altcoins follow bitcoin price movements and if bitcoin dropped then ethereum also drop and if bitcoin rises then all altcoins follow and same happen to ethereum. It is best to stay with your bitcoin holdings rather then converting it to ethereum.
Both are money markets and highly volatile so it depends on you where you want to enter. Crypto market is more volatile then share market but rewards are also huge in crypto market rather then share market. Share markets are controlled by laws of each country but crypto market is decentralized.
Bounty is not an investment because you can earn many coins here by just doing some works provided by the bounty and it require only your time but no money out of your pocket. Every bounty fixed a part of their funds to distribute their bounty workers and you get your share as per your work done.
As per my opinion, ethereum has highest potential of growth out of these three altcoins because ethereum have a big support of ERC20 tokens which creates a special value for this coin as mostly all ICO's using its ERC20 platform for distribution of coins.