ive never owned a big tank i started out with a 6 gal nano cube and it kicked me in the but quickly ... but thats a tiny tank .. if you do a 20+gal you will be alot better off, the more water you have the harder it is to screw things up ...you really need to do your research and find out what your looking for into a tank .. what you want for fish,corals etc ...then decide if the fish you want are going to need a bigger tank or if they can live in a nanocube, i ran a 1gal pico tank for 2 months .. i did 2 oz wc every 4 hours .. 24hours a day .. now that is high maintenance lol
ALSO i wouldn't buy a nanocube for just one reason ... you can buy a much nicer hand made ALL-IN-ONE tank for the same money ... i will send you the link since no hotlinking is allowed