Well, ultimately the decision is up to you.
Here's what I know about tamas
v3: They have good games!!! my friend has one and I luv the games that it has. They don't have many special features like jobs, and skill points in v4s and v.4.5's or tama familys and tv on v5's or musical instruments and bands like v6's so isome ppl find them boring. I've never had one so I can't say much about a personal opinion.
v4 and 4.5: these are the same things but with different jobs, different skill point categories, and different tamagotchis. I have a v4.5 and I love it! If you are not familiar with these you can get skill points in three different skill point categories. these affect your jobs and what your adult tama will be. in toddlerhood, you go to preschool and earn skill points. in teenagehood, you go to elementar skool and you can choose which catagory of skill points you get for going then in adulthood you get offred a job which gains you skill points and gets you $$$. games also give you skill points. i have found these to be very fun.
v5: these sound kewl, but my freind who has one said she didn't like it as much as she thought she would. You have to raise a famiy tamas and make sure they bond well with eachother. You also do things by watching tv. you by stuff on the shopping channel, you find mates on the dating channel etc...sounds fun 2 me!!!
v6 Music Star: Don't have one or know anyone with one, but think they sound cool. you get an instrument a toy and 3000p. then you end up joining a band, practicing w/ the band and giving concerts. I want one of these and I think they sound fun, but before i get one, I want to research other peoples opinions of them.
hope this helps!!!