Since you have a decent amount to get started with, and you want to learn how to manage your portfolio yourself, you probably should just open a Vanguard account and buy their funds. It costs nothing to open an account, costs nothing to buy the Vanguard funds from a Vanguard account, so all you pay is the expense ratio on the funds themselves (about 0.1% for index funds typically). With Betterment, you have to pay about 0.35% on top of the fund fees.
One thing that worried me about what you said:
However, I have heard about the importance of Asset Allocation and diversifying a portfolio into different funds so that on any particular year one can still remain successful.
Even if you have a well-balanced, well-diversified asset allocation, *NOTHING* will guarantee that you will be successful in any particular year. Trust me, even the most diversified investors lost money in 2008/2009. It's just a part of it, and you have to be ready for it and NOT SELL when that happens to you. In order to achieve a consistent average return of 7%+ in the market, you have to invest for the long term (15 years or more).
The people who get hosed, are the ones who get scared during a period like 2008/2009 when they see their portfolios lose half their value in a year, and then sell due to the fear. The ones who do NOT sell, well the value comes back -- in the case of the 2008 recession, you regained all you lost within a couple years. Now we're at new all-time highs again. Which means people will be itching to invest again (because we're at alltime highs, everyone's making money, nobody wants to be left out). Sooner or later, it will crash again, and those same people will get scared and cash out. Meaning they "bought high" and "sold low". Do not be one of those people.
You absolutely MUST have the fortitude to NOT sell, even if you have lost money over a year or longer period. Otherwise, buy-and-hold index fund investing is not for you.
