The funny thing about reefing is that most of us eventually always want to go bigger.

i think your determining factor is what kind of fish and/or corals are you hoping to keep? what is your dream fish/ what makes you want a salt water tank? You will be pretty limited in a 30g but the prices are more doable and mistakes are made on a smaller scale. Conversely, the smaller the tank, the more small issues can cause big fluctuations. In a larger tank you have more room for error- but mistakes are more costly. I started out as a newbie on a 220 gallon bc it was available to me and I wanted a wide variety of fish. It’s been great bc I have a lot of freedom for variety, but occasionally I have wished that I started smaller because everything is more expensive and costly than a smaller tank And mistakes are made on a larger scale. If your dream tank is 100g, I predict you won’t be satisfied with a 30 and you should save up and do it right. however, You could always start with a 30 and as you get more comfortable, plan for your 100g and use the 30g as a future sump or qt tank. Either way, as you move forward, here are my suggestions:
1. Save up and buy the best equipment. I can tell you I have spent much more money over time (and a lot of frustrarion) making up for my mistakes or 2nd hand equipment and inferior cheap products than I would have if I had just saved and bought “the good stuff” initially. If you start with the 30, you can likely repurpose most of your tank lights/sand/rock/power heads/rodi system etc in a later 100g and use the tank for a sump- so plan with a growth mindset and buy quality.
2. Have a plan of what fish/corals you want so you can purchase in order of aggression and know where you will be placing your corals in the tank.
3. Prevention will save you untold pain in the long run. Let your tank cycle well and completely. Set up a quarantine tank and use it for every new fish. Dip corals. Get test kits and monitor your tank. Set up a schedule for water changes and maintain it.
Hope this was some help!