So the OP is looking to cover local points of interests, things that distinguish this destination from that. I'll submit that you can cover those in Saigon in 3 days-- as long as you have three full days and the Presidential (Reunification) Palace isn't closed during your visit. Mr. Megatop and I decided that 3 days sufficiently covered what the city itself had to offer: the colonial and wartime buildings, the Ben Than Market (sp?), the shops and galleries, etc. We had four days and scheduled a trip to the coast, which gave us an opportunity to see the countryside but the coast itself was unremarkable. With still more time, we might have done a trip to the Mekong Delta, the Cu Chi Tunnels, or a resort farther up the coast.

Two days is not enough for Bali, not even close. We had five days and that wasn't enough. Bali is a HUGE island. Shaped like a big diamond with a little diamond attached at the bottom, it is about 70 miles N/S and 110 miles E/W at its widest points. Kuta can be skipped, but exploring the other coastal towns like Legian and Seminyak can take the better part of a day. Uluwatu Temple at the western tip of the Bukit Peninsula on the south (the little diamond) should not be missed for a good sunset and traditional dancing--that's the better part of an evening, perhaps followed by seafood on Jimbaran Bay. If you did a whirlwind tour in a speeding car for a day, you might cover a handful of the interior artisan villages but you wouldn't see much, and the artistic heart of the island in Ubud would be a blur. For all that, you haven't really gotten into the countryside to see the fields and volcanos, much less taken advantage of aquatic offerings or what your resort itself might have. Mr. Megatop and I had five days in Bali last May. We spent one day enjoying our resort to the fullest, then the rest sightseeing. We felt like we'd barely scratched the surface, as I said. So we're going back for another six days this May. Last time we stayed in Jimbaran. This time it will be Ubud and Seminyak, which will give us better access to the interior.

Of course, it all depends on what kind of holiday experience you want. But I'm sure that in selecting Saigon and Bali as your options, you've considered how different they are. In the time you've got, you can cover Saigon. You can't do justice to Bali in that time.