What is it that you are looking to see happen before you max out your 401(k)s? Wanting to "see how it goes" sounds like you are looking to see which account performs better over X period of time, which is precisely the wrong way to evaluate it. You need to be comparing fees, not chasing hot funds/stocks. Does your 401(k) offer a low-fee index fund (either a total stock market fund or a S&P 500 fund)? If so, throw all your money there and forget about it, because at your age, you have plenty of time to ride out the ups and downs in the market.
If you are really, really unsure and want handholding, then go compare the Betterment expenses against the expenses of the target date fund(s) in your 401(k). The target date fund will be managed to provide you a "glide path" (i.e., becoming more conservative as you get older), so you don't have to make decisions about asset allocation, etc.
A 401(k) has significant advantages to a post-tax account -- most immediately, it allows you to keep more money in your pocket now, because your investments are made with pre-tax money, so you can save more for the same net income; it also grows tax-free and has some creditor protection built in. That is almost the best starting point for investments.