I assume you are talking about building a starter's crypto portfolio. Best advice is to diversify into projects with good long-term potential and hold their coins. Look for projects that solve real problems and have steady development. Check DataDash's video on crypto portfolios:
https://www.youtube.com/watch?v=VD4XgHGrczcA great feature of both Bitcoin and Ethereum is their Proof-of-Work mechanism, that makes attacks prohibitively expensive and enforces a minimum price (cost of mining+equipment amortization). From a fundamental standpoint, Bitcoin offers more stability, security and transparency - innovations are implemented step by step, after peer review and community debates. That makes Bitcoin development to look slow compared to Ethereum, that has much broader functionality and complexity, yet surprises us monthly with unpleasant bugs or hacks. I hold Bitcoin, because of its limited supply, decreasing emission rate and market adoption. Factor in here the number of Bitcoin maximalists, that say "there is Bitcoin and altcoins" or "there is Bitcoin and all the other stuff". I also accumulate Ethereum, for trading purposes - trading ETH pairs was more profitable for me YTD and Ethereum protocol is expected to power many decentralized/distributed Applications (dApps) that are yet to come.
Bitcoin is at All-Time-High, double topping at 7700-7900 USD. A move to 9000-10000 USD is possible this year, yielding 15-25%, but it is certainly unclear if it holds. Bulls view upcoming CME Bitcoin Futures as a demand factor for Bitcoin, I expect only a short run, followed by a sharp correction. Derivatives drive markets to overvaluation - cryptomarkets are already overvalued, just like Dotcom was and introducing BTC futures to professional traders would only make more visible the discrepancy between the price of crypto and immediate benefits.
Ethereum is about 25% below its ATH and might revisit 400 USD area, going even higher, especially if at least 5-10% ICOs running on Ethereum platform succeed to deliver. Another demand factor is the transition to hybrid PoW-PoS consensus, as PoS benefits early/large holders and diminishes circulating supply. Unless Vitalik and his team of unicorn ladies surprises us with another major bug, as blockchain reversal, I see good market potential for Ethereum - it offers faster, more flexible transactions than Bitcoin, working smart contracts and better decentralization, mining being affordable even to individual users.
Pay attention to other projects from coinmarketcap.com Top-10 - Litecoin (technologically similar to Bitcoin, eagerly taking on innovations), Dash (fast, cheap transactions with great prospects of market adoption), Ripple (even better prospects than Dash), Monero (privacy oriented coin), Bitcoin Cash (if bought cheaply, a nice hedging instrument, yet questionable ethics).
How I'd build a portfolio from scratch? Find a licensed exchange working with fiat, deposit USD or Euros and buy the dip on each coin, until you have 50% BTC, 25% ETH, 5% LTC, 5% DASH, 5% Ripple, 5% XMR and 5% BCH.