Which snake? lizard?

[Black_Panther]

Though I have kept (and still keep) frogs, I am totally new to reptiles. I am interested in getting either a lizard or a snake (I like them both!) but seeing as I'm new, I'm not sure what to get. I'm looking for something that won't be too difficult for a beginner, isn't overly expensive to buy/take care of, can be held, and can fit comfortably in a forty gal. long glass terrarium. If it's a snake, I don't want anything that gets much longer than three feet max. I like to do a lot of research on any pet before I buy it, so I don't want to walk around the petshops and buy anything on impulse...

Anybody have any ideas?