I spent 7 years on Guam with trips in and out of Saipan. That was quite a while ago. At that time Guam was the honeymoon destination for Japanese newlyweds but Saipan was starting to get some of them as well as the tiny island if Rota. Saipan has always had their radar on to increase tourism and have spent some cash developing it over the years. Their primary targets were Asia but I am sure that if Australians indicated they might like that possibility, Saipan will retool for the down under tourist. I have never know Saipan to be much of a surfer's magnet tough and that won't help. Diving and snorkling are great on the reef side. The other side being the Marianas trench gets a little dark. Same for Guam but I think Guam has more beaches and a little more surf.
I still have a few friends between Guam and Saipan and if you want to talk about drugs and prostitution then look more at Guam. Guam has been sitting on 45% unemployment leading up to the pandemic whereas, Saipan went after the pandemic and didn't have the outbreak Guam did. I can certainly see some of the dealers and girls heading to Saipan if that is where the business is but from what I know of the 2, Guam's law enforcement could not stand up to Saipans. Not saying Saipan has great law enforcement but much better than Guams.
Guam is also a US military island. Air Force base, Navel station, Navel repair facility, submarine base, Navel communications base, Navel Air base, as well as the US Army building on it. If there is a club, there are military personnel. Massage parlors are full of military personnel. The military is Guam's biggest economy. Not so Saipan.
If there ever gets to be serious competition between the 2, it will get ugly. Guam and Saipan natives goes way beyond just not liking each other. Guam was US territory before the Japanese invaded in WWII. Saipan was Japanese territory from before the war. After the Japanese took over Guam they brought in community leaders, overseer's, and spys from Saipan. Bad blood until this day.
Of the 4 close Islands, Guam, Rota, Tinian, and Saipan, my favorite was always Rota. 64 miles from Guam and you fly on a 16 passenger turbo prop. One road the goes around the island but many paths to drive down if dry. A lot of WWII reckage still lays hidden in the jungle. A natural pool that raises and falls with the tide. There is an all inclusive Japanese hotel on the island but the visitors never seem to leave the property except on some kind if road safari that just traveled along the one road. Outsiders do not get in either. We tried a few times. There was another hotel. At that time it was the Blue Peninsula. A friend owned it at that time. Had a restaurant where if you brought in your catch, they would cook it. Sometimes those catches were lobsters grabbed right off shore. Otherwise they had their own really good food. Rooms were cheap and the sort if place everyone hung out in a veranda in the evening going into the early morning hours. Beer was always cold and plentiful. More suited for the island vagabond traveler. One bar besides the Blue Peninsula on the island. Everyone is really friendly.
Tinian has little to offer besides the runway where the Engola Gay took off from to drop the A bomb on Heroshima. Otherwise they have the only fresh milk opperation that services the islands.
I can see Saipan picking up some Australian travelers but their money will still come from the Asian tourist. I can't see Bali getting too hurt by Saipan's push. Well, as long as this nation can get their shit together and get the nation vaccinated. I think travel cost will play into it also. Will cost a lot more to get to and from Saipan than it does for Bali. Plus, cost of goods is a lot higher in Saipan. Everything is shipped via container into the island with some perisable food items air freight. I remember when prices in the Mariana's islands were close to 5 times those of the US mainland. Hawaii even has cheaper cost.
Puspa lived in the Marianas islands and now in Hawaii. Maybe she can add more insight.