I prefer the V5. It's easier to care for and you could leave it alone for hours and it won't die. A lot of people say it gets boring, but trust me, it's not. (At least to me it's not.)
i like my v4.5. it is more challenging because you might get robbed and lose your points and its better suited for people who want to buy items and foods in the shop with their earned gotchi points without using the cheats which takes away the fun. you also have to play games to keep their happy hearts full while on the v5 you could easily fill the happy hearts with the rare items in the chest icon.
I like both. V4/V4.5 and V5 are very diffrent and thats what I like. V4/V4.5 is when you raise your Tamagotchi to get a good job while V5 is raising a family! I love both!