Mark Hamill expressed some surprise that anyone could manage to make a droid cuter than R2-D2, whom he considered "the most adorable droid in movie history."
I gotta call him out on this one.
When Disney bought this franchise, the responsibility for creating Star Wars droids was handed off to the people who made WALL-E. WALL-E, for gosh sakes. R2-D2 doesn't stand a chance against that guy for cuteness.
Consider this famous sheet detailing the features that make a cartoon character "cute"--large round head and eyes, rounded chubby features, and so on. Disney invented the principles of cuteness (or at least wrote them out for others to reference), and yes, BB-8 is objectively and measurably cuter than R2-D2. He is, essentially, a baby astromech. Even his voice is designed to sound less like sterile electronics and more like baby talk, and his name invites the same comparison.
But if I were going on an actual space adventure, and I could only pick one astromech for my team, I'd take R2-D2. Because BB-8 may be adorable, but R2-D2's a badass.
BB-8 really isn't much more than a mobile, sympathy-inducing Maguffin container. He's pretty helpless. He's got a shock probe, but that doesn't prevent him from getting a net or bag over his head. He provides some information to the protagonists, but mostly he's just along for the ride until his map can be read. Although he's presumably a perfectly competent astromech copilot (worthy of Poe Dameron, no less) and a good watchdog, he doesn't get much screen time in those roles; these are purposes any astromech could have filled.
On the "reality ensues" front, anyone who has one of the little BB-8 toys knows that his design is prone to getting stuck in low spots and picking up dirt, especially when navigating on sand. He probably has to maneuver very carefully to ensure that the tool he needs isn't stuck underneath him, or right under his head. When a crisis happens, his only real response is to twitch and scream a bunch of beeps that probably translate as "Help me Rey, you're my only hope!"
By contrast, R2-D2 gets things done. In every emergency he scoots straight over to where he needs to be and fixes the problem.
Naboo cruiser under attack? Head out and fix it while every other droid gets blown away. Enemy droids? Burn them with oil and rockets! Inconvenient doors? Hack 'em open! Restraining bolt? Sweet-talk Luke into removing it and continue the mission, even through sandpeople territory. Death Star tractor beam? Scan for the controls, and find the princess while we're at it. Trash compactor? Shut them ALL down! Fell into swamp? Periscope! Swallowed by swamp monster? Personally, I don't think it was a matter of tasting bad; I think R2-D2 shocked it. Hyperdrive's been broken all episode? Fixed in two minutes. Need a lightsaber smuggled into Jabba's sail barge? He'll get it done. While everyone else is panicking, R2-D2 stays calm, carries on, and solves things literally under everyone's noses.
Reality ensues for R2-D2 as well, of course; he has the benefit of mostly-flat surfaces to roll on in the movies, and doesn't move all that quickly. He's vulnerable to electrical overloads and Jawa kidnappers.
BB-8 may come more into his own when he grows up a bit and gets a full trilogy of screen time, especially if he gets more time in civilized, technological settings where a droid is more at home and has more things to interface with.
And yes, you may call me biased for being old enough to remember when there were no prequels, and yet young enough that I first encountered R2-D2 in a Sesame Street appearance before I realized he was affiliated with any particular movie. To me, he was a Famous Robot From Somewhere before I knew him by name. I won't deny any of it; my answer stands. This spaceship has only one astromech slot, and R2 gets it for now.
