I reckon eff 7 is better because i reckon it has more uses than a Melon sword and it does a lot more and earns more money than i reckon a Melon sword would.
Melon sword by far. I'd rather have a safe way of making a set amount of money each hour. An eff 7 doesn't really give you a set amount of money per hour and it isnt all that safe without pyro+/fly+. I've had both tools, and me and the person i share them with have both decided we are going to sell the eff 7. Reason were selling it is because it is way over hyped and not worth the money. Also melon farming allows me to watch anime, so im not sitting there bored for an entire hour.
I currently own a melon sword and find it a very consistent way of making money. I have never owned an star pickaxe but I think it is a bit over hyped. But I plan to have both.
Eff7-more money per hour, much more work Melon Sword, quite a bit less, much less work (you don't even have to look at the screen) I find it good to have both.
I like a melon sword better, because based on the times that I have rented one, it makes 12-14k per hour, and I'm watching YouTube while farming. Eff7 may make more, but 1) Without fly+ and pyro+, there is a chance you may die, and most likely lose 500-600k and 2) You have to pay attention to the screen. Melon swords don't require any donations (fly/speed is nice), and if you die with a melon sword, well.... G_G. My Vote: Melon Sword
Eventhough I don't have any of the tools listed, I do believe a Melon Sword is the best Star Tool, then the Star Pickaxe afterwards. EDIT: I currently own a melon sword, and it as a very simple and easy way of making money. Eventually, I plan to get Pyro+ and Fly+, following an Efficiency 7 afterwards.
I personally like Eff7's more, but you could have a really strong debate on this from either side I believe. Eff7 - More "fun" and can make you a lot more money, but it is recomended to have pyro+, fly and Auto Ereps as these are very easy to loose, drop, break, burn or die with while mining. Melon Sword - These make a very steady amount of money per hour, only feature recommended with this is Auto EReps as Melon Sword's also break easily, (but not as much as an Eff7, because there are no distractions of mining) but can tend to be a bit boring for those who aren't big on farming. Ehh.. nevermind. Eff7's win. ;D