Alright guys, in order to answer this question, we need to analyse each character, and compare them.
First off, Some context.
Sans is a goofy skeleton who lives with his brother Papyrus in the quaint little town of snowdin. While at first, it seems that the full extent of is abilities is telling bad jokes, over time, he is shown to posses an immense amount of power. However, it isn't until a player attempts a genocide run (killing every character in the game) that true scope of his capabilities are revealed. Near the end of a genocide run, he passes judgement upon the player for their sins, and proceeds to give them the ass-whooping of a lifetime.
Now that you have some basic info, here are his actual strength and weaknesses in battle:
Gravity manipulation (telekinesis?) - Like his Brother papyrus, Sans hurl the player about the area. It is unclear whether he is simply using some kind of telekinetic magics, or if he is actually changing the gravity affecting the player. I'm going to choose the second one, as it makes for a more interesting fight between the two.
The Gaster Blaster - Sans can summon an object resembling a canid skull capable of firing a massive beam of energy. The resulting attack is probably equivalent to an exhaust plume from a rhino engine
Dodging/Short Range Teleportation - Sans is shown to be able to teleport himself across short distances, no more than ~30m. He can use this ability to dodge just about anything thrown at him, unless he happens to be caught off guard.
Throwing bones - Can throw bones. Lots of 'em
Karmic Retribution - Arguably the strongest of his abilities, and the one that makes the least sense in a game like KSP. Karmic retribution removes the player's invincibility frames, meaning that every frame that one of his attacks is in contact with the player, it converts 1 normal hp into 1 poisoned hp. While that may not sound like a lot, it's worth noting that Undertale runs at 60fps, meaning that a player with the maximum amount of health (92) can be reduced to the brink of death in less than 2 seconds. As for how the poisoned hp works, I never figured out the specifics, but it seems as if the more poisoned hp a player has, the faster that hp decays (like uranium, and other things with half lives!). BUT, as if this ability wasn't already complicated enough, it's actually strength depends on how evil the target is. Since by the time you fight Sans, you're basically Satan incarnate. As for how "evil" Jeb is, I have no idea, so we'll just have sans be as powerful as he is in his boss fight
Awareness of timelines and Save/Load cycles - Although it's unclear whether or not sans can actually see into other timelines, he is definitely aware of their existence. This basically means that Sans will be able to figure out if Jeb is quicksaving/loading
Puns: He's got a ton of them. A skele-ton
Low stats - With incredible stats of 1 atk - 1 def, sans is technically the weakest enemy in the game.
Terrible Endurance - Sans cannot keep up a fight for too long before he might actually fall asleep. ~15 minutes before he has to take a rest
Nihilistic Viewpoint - Sans' awareness of save/load cycles have made him just about give up in life. The only reason he fights the player is because they threaten to end ALL timelines. This won't affect a fight much, since we're assuming he has a good reason to fightJjeb, I just think it's a cool part of the character
Jebediah B. Kerman ("Boosters" is his middle name)
If you're posting here, you probably already know all about KSP's golden pale green boy. You probably also know that you really can't do much with only 4k funds. That's only enough to afford 10 RT-10's, and that's ignoring any guidance systems to make it useful in a fight. SO, instead of a wimpy 4k funds, I say Jeb gets a more reasonable 100K funds to use, and 30 kerbal days with all the info listed above to prepare for a more fair fight. Now then, since KSP is a space exploration game, not a fighting simulator, Jeb is essentially limited to blowing something up something as far as attack methods go. Lucky for him, KSP has no shortage of ways to make things go boom.
1. Missiles: The first thing that comes to mind is a short range ballistic missile, like the ones we all used to make on accident.
2. Bombs: Though there aren't too many ways to build bombs, they are still a viable option
3. Some sort of specialized attack vehicle: Pretty much anything else he can do is just find a different way delivering one of the first 2 items to the target, whether it be by plane, tank, rocket, orbital bombardment structures, or an ion powered solar glider.
Basically it all boils down to Jeb coming up with a way to make a lot of "boom" happen, without giving sans a chance to teleport to safety. He could blanket an area with so many explosions that sans doesn't have a chance to dodge it, but there's nothing stopping Sans from just slamming whatever Jeb built into the ground, or sending it on a nice Kerbin escape trajectory.
Given enough funds, Jeb could probably manage a hit on Sans, and a hit is all it takes, but at 100k, it's unlikely, and at 4k, it's practically impossible. Sans probably wins, although it depends on what Jeb can come up with.
Honestly I think his best option is invoking the Mighty Space Kraken for help, which makes me wonder...
How would Gaster hold up against the Kraken?