Okay, I think it's about time a JBRoomer came in (and wish it would have been sooner, in retrospect)
Now, I don't have much Sonic experience. The only Sonic's I've played were Damax back at ToS5, and I play Infzy fairly consistently (But I usually pick Wario) but from what I have seen firsthand, and a healthy dose of theorycraft and other player's experience, I think I can see both sides of the argument. Mind you, there is always a bit of character bias in almost every post, but I will try and look at this objectively. (
Try to

)
Range. Jiggs range is poor, but I think you're underestimating her bair. It has surprising range, more so than our fair, if you can believe it. Sonic, however, has access to a few disjointed moves (uair comes to mind) that can stop us. Now, the inherent problem with these moves, is we will try our hardest to not be in position of them, to the point that you could control us knowing that fact. Now we're getting into mindgames, but chances are we won't be above you unless we're dair resting you. Do I think uair is unusable? HELL NO. Just bear in mind that we are going to be approaching you from an upwards angle, and we will do our best to never be above you unless we're waiting for spring vulnerability. You can try and exploit than mind set as you see fit. With the exception of these disjointed moves, our range is actually quite comparable. That's where maneuverability comes into effect.
See, that's the biggest thing about this matchup. On the ground, you completely outmaneuver us. To a ridiculous degree. In the air, it's the same boat, but reversed (obviously) Now, you will go in the air, we will go on the ground. We have to reset our jumps eventually, and almost any of our moves will pop you up into the air. Our WoPs can be screwed by your ridiculous running speed and pivots, but we can lay a brickwall bair cycle (which I love to do) to keep you away from punishing. We're pretty much at a stalemate as far as this goes, which quite frankly sucks for us, as we're used to constantly weaving, and I'm pretty sure you guys are in a similar boat. Well, what would swing this in any other favor if we're at a maneuverability stalemate?
Killz. That's what. Well, this can be broken down into two parts. One, our killing power. Two, our survivability. Our killing power is significantly above average. We are pretty much the definition of a glass cannon character. We can kill with fair at 90%+, your position depending, obviously. Our Fsmash can own a techchase/trip scenario, and our Dacus kills if fresh, but chances are that won't be able to hit in this matchup barring extreme mindgames. Rollout kills well before that, as does rest. Rollout is fairly hard to hit with, Rest isn't. Yes, you read that right. The thing about Rest, and this is really prevalent in this matchup, is it interrupts everything. Everything. With perfect reflexes, the only two moves that can't be rest countered are Shine and Rest itself. (I believe that's what Veril said.)
That's with perfect reflexes, but once the timing is learned for Sonic, his moves actually aren't very difficult to interrupt. I'll leave the details to Veril, but any multihit move can be rest interrupted. Any move where you extend your hurtbox with your hitbox can be rested. Unfortunately, that is a lot of your moveset (hilariously enough, Jiggs falls victim to herself in the same regard). If you're at killing percent, simply approaching us can be deadly. But I stress the "if you're at killing percent" part, and I'll return to that later.
Onto survivability. Surprisingly, Jiggs has fairly good horizontal survivability. In tourney settings, against high level players, I'll stress, I've had her live past 200%. Our dair is xrazy for Infzy DI, and our horizontal speed and acceleration allow us get back quickly. Now, obviously that limit WILL drop. But we don't die at 90% as everyone seems to think, in fact, I'd wager that we can kill you a heck of a lot earlier than you'll be able to kill us. Horizontally.
Vertical death is another situation entirely. Remember when I said that we'll be avoiding your uair at all cost? Well, that's because Uair is the move that will net you the early percent kills if you can nail it. Work on ways on hitting it, that's the key to beating us. Now, you have to balance using it and staling it, like many great moves, but I would recommend keeping it fresh and attempting to beat us by constant retreat and catching us in our retreat. This match will be very back and forth, constant traps in effect.
Now, here's the one thing I don't know, and would effect this a lot. I have no idea how much damage Sonic's moves do on an average hit. I know Jiggs, and I know that she does over 10% for every one of her most used moves. Considering we kill you at ~90% with a fresh fair, and I think ~75% with a Rest, that's 8 to 9 moves then a lost stock. Getting those moves will be a chore, but that's what I see. Mind you, these are using rough estimates and data supplied, I don't know the proper DI kill percents for Sonic on her moves. Take that data with a grain of salt.
Now, where does this lead?
This match is so back and forth, honestly, I call it neutral. It's all about the player, I feel the character strengths effectively eliminate each others'. I personally feel it's 55-45 Jiggs, and I concede that you probably think it's a similar ratio for Sonic. It is for that reason, taking into consideration what I know about both characters, that I would call this one dead neutral.
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