Well, from someone about to put her foot in her mouth.... Both. Eventually get both.
Get the IDL first. Many of the items are buyable and are still happy items (Grown, I think...) on the 15th Anniversary. I'm on Generation 42 or something like that. I haven't put it down. I love it so much. It VERY fresh to the people who haven't had a color tamagotchi yet.
The only real down side is when they start randomly saying something to you (or another character) and you can't read it...
Meanwhile, I love the characters a bit more from the 15th Anniversary version. I love when you connect between two tama colors... The game is cute, friendship levels - when they go up, they both let you know, Random gifts..... (In some cases, very random) I've basically bought/grown all the items on my IDL and sent them to my 15th Anniversary version. ...