They are the same visa. The visa on arrival and the e-VOA both give an American the same B1 tourist visa, at the same official price, for the same 30 days. The only real difference is where you do the paperwork: at your kitchen table before you fly, or at a counter in the airport after a very long journey.
What you get either way
The visa on arrival and the e-VOA are two ways of buying Indonesia's B1 tourist visa, and the result is identical. Either route costs IDR 500,000 and gives a stay of 30 days, extendable once for another 30 days. The same rules about what you may do in Indonesia apply to both.
What the letters mean
VOA stands for visa on arrival. The "e" in e-VOA means electronic: the same visa, applied and paid for online in advance. Indonesia's official e-visa site lists it for US passport holders as "B1 - Tourist (Visa On Arrival)". If you see the two terms treated as different products with different prices, you are probably looking at an agency's page.
The case for buying online first
Buying the e-VOA before you fly moves the payment and form-filling out of the airport, and the State Department notes that travelers who wait to buy at the airport may face long lines. There is no nonstop flight from the United States to Bali, so the visa counter comes at the end of a long journey with at least one connection.
How the online route works
You apply on Indonesia's official e-visa website, run by the Directorate General of Immigration, and pay by Visa, Mastercard or JCB card. One application can cover up to five people, which suits a family. The application asks for your departure date and a booked return flight. Once approved, the visa is sent as a download link by email.
How early to do it
The visa is valid for entry for 90 days, so it can be bought well ahead. Our advice is to do it once your flights are booked and your passport details are final, leaving time to sort out any problem with the application before you travel.
What can go wrong online
The main risk online is the wrong website. Third-party sites may charge a fee on top of the official price for the same application. Use the address printed on the State Department's Indonesia page, and check that the price you are asked for matches the official one.
The case for buying at the airport
Buying the visa on arrival at the airport needs no preparation, which makes it the fallback for a trip booked at short notice or an application left too late. The State Department confirms that a traveler can purchase a visa upon arrival at the airport, at a booth located before the immigration counters.
What it costs you
The price is the same. The cost is time and attention at the end of a long travel day: a payment counter first, then the immigration line.
When it is the right choice
The airport route makes sense if you could not complete the online application, if your plans changed at the last moment, or if you are uneasy about entering card details on a website you are not sure is the official one. It is a perfectly valid way to enter Bali.