Saltwater newbie, 120G or 36G?

I always say that the "Bigger the better", but bigger tanks have their problems too. More light, more LR, more expensive WC, ect.

If it a reef or FOWLR??

PS--If it was me, I would go with the 120.

120 for sure if you have the budget for the equipment, would suggest picking up a 55g for the sump. If you go with 38 and decide you want to upgrade later a lot of the equipment you got for the smaller tank will be inefficient for a larger setup.

Larger tanks are more forgiving. More water = more dilution meaning less Ph/salinity swings if you make a mistake.

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Pick what animals you want. That, more than anything else, will drive what will make you happy. Yes, a 120 is going to cost more to setup, but if you want larger fish, or lots of fish, it's your only option. I don't think a 120 is really a huge tank, tbh. A smaller tank, if setup and stocked properly, won't be as much effort, just more frequent attention.

I agree, with a FOWLR, I would use the bigger tank. Most SW fish get bigger then FW and would like/need the bigger tank. With a FOWLR, the cost if not "that much" more. You would not need the DI/RO water, a sump is not "really" needed, no calcium additive or calcium reactor, no special lights, ect.

8 to 10X would be on the extreme low end even for a FOWLR. Granted you don't need the 40 to 60X turnover that a reef with more difficult corals would demand but I would still want at least 30x turnover. Flow isn't just for corals, it for overall tank health. Good news, is this flow can be accomplished with powerheads. Even if you had a sump, you wouldn't want that much flow going through it.

If this were a reef, I would say the cost of a 120 v. a 36 would be more of an issue. However, since you have the tank and you intend it to be a FOWLR, the major difference in cost is already behind you. Lighting a 6' FOWLR isn't going to be significant over a 36. Similarly, a skimmer (if you decide to get one), won't be a significant difference for the larger tank. The biggest short term cost increase will be the amount of rock you need to buy (although dry rock can help offset that a bit). The major long term cost difference will be maintenance costs. If you are going sumpless in a 120, you will probably want to do weekly water changes. At least bi-weekly. Also, those water changes will probably need to be of the 20%+ volume versus mainly 10% changes on a sumped system with a good skimmer.

I am a big proponent of using a sump. There are just so many more options available and benefits to having one.

I may be way off but many people start off with a FOWLR, only to want a reef down the road. I suggest you take a stroll through a good LFS or visit some online retailers and make sure you want a FOWLR. The costs really start to add up when you buy equipment twice or have to add equipment to supplement the lower end stuff you start with. Sounds like you have a bit of experience under your belt. A little research up front and a commitment to do things right and I have no doubt you can handle a reef.

I'd go the opposite--the smaller the tank, the better off you'll be having a sump. In a bigger tank, the sump is primarily to hide equipment and push water around, but in a smaller system, the additional water volume is really helpful.

Either way, sumps are good. They make a lot of things much easier.