Does anyone have an idea about which is the best service when it comes for providing API's to help retrieve info such as balance, transactions, etc?
I was thinking about using Blocktrail's one for BTC and Etherplorer for ETH but I'm not sure whether they're better alternatives or not.
What about Blockchain.info? Do you want API's accessed by just the endpoints or created for a specific programming language? To be honest, most of the third party API's come with restrictions, if you're looking to develop a commercial application, you should use the JSON RPC and that gives you more control over the API's.