With either method, coverage costs the same: rates are legally regulated. Therefore, it's all down to what method you prefer.
Fully online buying offers more convenience, but you can also get into big trouble if you don't understand your options--what if something happens and something you thought was covered isn't? Insurance agents study the industry and typically have more experience than the average person in it.
The ideal is the best of both worlds: online shopping combined with personalized support from a broker. That is available from some sites. Just try to go through a specific brokerage and stick to one agent from them instead of an aggregator server somewhere--the latter are the ones that give your information to multiple dealers.
Source: http://www.vitalonehealth.com