in my opinion, it's more cool to get a small juvi/baby bichir. It nice to see one grow from a puny fish to a monster of 2 feet. anyways, you'll have more time to enjoy ur bichir. Babies will be more attached to you once you take care of them until they reach adult. i know a bunch of guys that are against me in this though! LOL.
IMHO it's a matter of personal preference. Some people want the instant satisfaction of owning a monster while others like to spend the time and effort watching a juvenile/baby grow into a monster. Good luck in whatever you choose.
i personally prefer to buy the bigger ones. i think it is much cheaper that way considering the amount of money & time you need to spend on food and maintenance for them to grow to a large size.