I lived in Phu Quoc for several months during 2020 and visited Con Dao for a week. So I have a good idea of both places and can tell you they're both completely different.
The two images below illustrate the contrast between Phu Quoc and Con Dao. Most of Con Dao's coastline is natural and undeveloped. While almost all of Phu Quoc's coastline is developed.
Difference in Size
Phu Quoc is roughly 574 km2 (222 sq mi), while Con Dao is 75.15 km2 (29.02 sq mi), almost a tenth the size.
Where are the Islands?
Phu Quoc and Con Dao are both in Southern Vietnam. Phu Quoc is off the coast of Cambodia in the Gulf of Thailand, while Con Dao is in the China Sea off the Mekong Delta coast.
Getting to the Islands
Getting to Phu Quoc is much easier, and there are more routes than Con Dao.
Both islands have airports, with regular daily flights from Ho Chi Minh City. Phu Quoc's International airport offers flights from many local and international destinations, while Con Dao only connects to a few domestic regions.
Phu Quoc has an excellent ferry network connecting you to mainland Hà Tiên, Vietnam. Con Dao also has a ferry, but it takes between 4 to 6 hours.
Hotels and Accommodation
Phu Quoc has exploded in the last decade.
Almost the entire 30-mile western coastline is developed with hotels and resorts. And, they're still building more!
There are tons of accommodation options in Phu Quoc. You'll find everything from budget hostels to many luxury resorts.
With Con Dao's sensitive biodiversity and conservation programs, it seems they're a lot more conservative with property development. Most accommodation is in the city, with only one or two luxury resorts on the coast.
Although accommodation isn't expensive in Con Dao, there are fewer budget options, but a lot in the $25 - $50 per night price bracket.
If you're looking for beach experiences, Phu Quoc has many more beaches, including beach amenities like restaurants and bars.
If you want to escape the crowds, then Con Dao will definitely be the better option. Con Dao's beaches are much cleaner than Phu Quoc's too.
Things to do
Both islands offer similar experiences, but Phu Quoc has more of everything.
If you are traveling with kids or want a party vibe, then Phu Quoc will be your better choice.
However, if you're after a Vietnamese wildlife experience, love scuba diving, snorkeling, fishing, and hiking, then Con Dao is, by far the better choice.
Phu Quoc has some fantastic restaurants, with something for every preference, including many vegan and vegetarian options.
In Con Dao, food options are limited and slightly more expensive than Phu Quoc. The restaurants mainly cater to Vietnamese and Chinese palettes, and you'll have a tough time if you're vegan.
If you're looking for a party, you're spoiled for choice in Phu Quoc. There are loads of beach bars, nightclubs, and even the off music festival.
Con Dao is more conservative. There are one or two bars, but they're very tame and are usually closed by midnight. There's a strong military presence in Con Dao, and you get the sense they wouldn't tolerate a rowdy party vibe.
I loved Phu Quoc and Con Dao. I'd say Phu Quoc feels like a typical tourist destination, whereas Con Dao is more authentically Vietnamese, with an old-world charm.
While both islands offer scuba diving, but Phu Quoc doesn't even compare to Con Dao. Phu Quoc's waters have been overfished, and the sheer volume of tourists means reefs are damaged and bare.
Con Dao has a much better approach to eco-tourism. They limit guests and the frequency of visits to marine attractions. The waters are clean, and marine life thrives.
In Con Dao, you also have the option of a Bay Canh turtle tour to large turtle species like the Hawksbill sea turtle, Green turtle, and leatherback turtle.
In short, if you're looking for a party, then I'd go to Phu Quoc. If you want a quiet holiday and exploring nature, then Con Dao is the better option.
If you have the opportunity to see both, then absolutely go for it. Phu Quoc and Con Dao are very different, each one offering a unique island experience.