It really depends on what your involvement in cards is.
If your goal is to only collect the inserts for one team, then go for single cards.
If you dont mind collecting other teams as well then single and boxes are a good idea.
Boxes are fun, especially if you buy from a reputable seller with unsearched cases, then you never know if you will hit big or not, kinda like gambling but more fun.
Cases are a lot of fun, but a hell of a lot of work. The plus side is you get a huge assortment of hard to get cards, and pretty much garanteed the "one per case" hit. The down side is they are very expensive, and it takes a LOT of time to sort out common cards. But you could just give them away to kids or donate to a charity. I bundle mine into team sets and sell on eBay, it takes DAYS. Ive only opened 3 full cases (2012 NRL Champions, 2012 AFL Champions, and 2009 NRL Classic) so im still in the learning process, there are guys here who have opened a lot more cases than me and can give more refined and informed advice.