I prefer Mario Kart. Most Sonic tracks have side guards everywhere (like in MK 64 Rainbow Road) and touching them barely slow you down, not to mention the snaking.

I like Sonic & All-Stars Racing Transformed better. By quite a bit. Reason is, Mario Kart 8 was overated for crap like the anti-gravity which was nothing new. Crash Nitro Kart literally had it where you'd jump into the air or something and the wheels did exactly what Mario Kart 8 did and turn sideways. Attracting the surface. Just annoyed me that people ignore these things and act like MK invented something new. But anyways, I prefer Sonic for a few reasons. I like the items better though they needed a few more (loved the swarms idea. Way better than a blue shell though it needs a stronger effect). I think the courses just look more appealing and when people see me playing (because I was like top tier in it reaching 11 on leaderboards for like the first year of it's release), they'd always go crazy like this game looks crazy as hell. And so on. My last thing is kinda biased and my main reason. My 2 favourite Kart Racers of all time were Diddy Kong Racing and CTR: Crash Team Racing (I've played every MK but just not as good imo). So my thing is Diddy Kong Racing's feel is all over Sonic Transformed, with car/kart, boat/hovercraft, plane. Second is CTR, this games items can sabotage like any Mario Kart, but computer players I've noticed can not do Lead target weapon spamming lol. But the key in this one is item carnage with the speed, crash team racing is about doing mini boosts with the drifts, jumping off jumps for boosts and so on, so I can boost basically none stop throughout a race and computers can keep up off skill. That's also in Sonic Transformed, it's about boosting as much as possible, avoiding weapons and comps on harder difficulty don't sleep lol.

i also have both and like to play online most the time but nobody is playing sonic transformed online so the winner for me is mario kart 8

the artstyle of many track in sanic and allstar racing is realy nice. i like that part better. the rest, well mk8 is just epic and Sanic has a lot of flaws.

Have you played 200 cc?

MK8 did it much better. It's like saying Galaxy copied off Sonic Extreme... Mario Kart 8 had some crazy level designs thanks to the anti-gravity feature. You also get speed boosts if you bump into each other while driving on these anti-gravity sections. Crash Nitro Kart didn't have the crazy level designs or the speed boost mechanic. It was a half assed feature in the game.

Being able to spam boost drifts isn't a good thing by the way. Mario Kart DS and MK: DD had that and most people are glad snaking is gone.

Yep. I wouldn't say sasrt is bad, it's just inferior to mario kart in every aspect.

Because it doesn't work for Mario Kart... Which is why I don't like it as much. Snaking is a tactic that wasn't an original intent. Rewarding you for choosing to drift a corner, doing a jump or fliping your vehicle is entirely different than snaking. Hell snaking doesn't have any advantage in those either. 
Mario Kart isn't the same game as those, they work with different mechanics. Look up some videos and see if Sonic Transformed or CTR is bad because of those. 

Also doing antigravity better than another game doesn't give the right to act like you're the first to do it. Which everyone acted like. How well it's done isn't relevant. Mario can mimic anything and it's fine Sonic Transformed was attacked for Copying MK7 lmao and it wasn't even close, it actually could be said it was copying DKR though but ppl didn't really, but MK8 does something exactly like in another game it got a pass lol. My point.

Mario Kart is better overall

But Sonic ASRT is still good

Mario Kart 8