What to level up is a difficult question as it depends on many factors, such as your long-term ambitions and ability to deploy capital.

I'm assuming you're considering buying cards on the market, to combine and level up. In general I think this is a better strategy than buying booster packs, because you can get only what you need at exactly the right quantity. Check out the @peakmonsters market at https://monsters.steempeak.com/market, which has a more advanced interface. Also consider @steem-plus for bulk buy on the @steemmonsters dot com market.

That being said, I would suggest leveling up two splinters to start with. Since you have to choose a splinter to play with, it's better to have 2 maxed out than 5 mediocre. Other advice for leveling up is to first focus on neutral cards and summoners. Neutral cards because they can be played with any summoner. Summoners because without them, the level of your monsters is limited. Selenia Sky, a dragon summoner, is also a good choice, since for under $100 USD you can get a max-level summoner that can summon any monster (given the appropriate rule format).