There are differences between just keeping live rock (we call this Fish Only, With Live Rock, or FOWLR
) and a reef tank.
First and foremost would be lighting. Coral require intense lighting to live (actually the photosynthetic zooxanthellae that live in coral require lighting, and in turn provide food for coral as a byproduct of photosynthesis). FOWLR
tanks can get by with much less intense lighting. But reef tanks will need fixtures that can get quite expensive.
Second would be flow. FOWLR
tanks need far less flow than corals need to live.
Third, water quality: corals require pristine water conditions to thrive. This means additional attention has to be paid to removing nitrate and phosphate, usually via protein skimmers. Also, other water parameters have to be maintained, such as stable alkalinity and calcium, and magnesium. Other elements also need to be present, though they can usually be maintained via a good salt mix. It also means you might have to stock less fish, since fish produce waste that is eventually converted to nitrates.
Reef tanks tend to cost much more, both up front, and over time, since extra equipment and some times extra additives are required to maintain water parameters. Also, coral can be quite expensive, and quite finnicky. It requires careful monitoring and slow adjustments. Something like a power outage that lasts a few hours can kill coral, as even minor swings in temperature, or loss of flow for a short time, can damage or even kill coral. Therefore provisions need to be made to provide power for heaters and powerheads during an outage.
Other considerations are sumps: It can make life a lot easier to run a sump on a reef tank - it will both increase your overall system volume making water more stable, but also provides a place to house equipment. It is much easier to do a reef with a sump. FOWLR
tanks still benefit from a sump addition, but not as much as a reef tank.
I think a 60gal is a good starting size, for FOWLR
or Reef. I myself started my first reef tank about a year ago, and it is a 57 gallon tank.