1. Mos Eisley Spaceport is the most wretched hive of scum and villainy ever
Mos Eisley is a sorta scummy place, I guess - there are some rough characters there who would pull a blaster on you at the drop of a hat (not noble hero Han Solo though. No, he would only pull a blaster to defend himself). However, Obi-Wan telling Luke he would never find a more wretched hive of scum and villainy was more than a little disingenuous, considering there's a FAR more wretched hive of scum and villainy located ON THE SAME PLANET.
That's right - Jabba's palace, a hive that's 20 times more wretched and scummy and villainous. There's a fat slug guy who keeps a human being frozen in carbonite as a conversation piece, regularly abuses his slaves, and has no problem feeding his servants to the giant cave monster who lives in his basement. Also, his idea of a good time is throwing people into a toothy sphincter in the middle of the desert.
Compared to that, the Mos Eisley Spaceport is practically a neighborhood Applebee's.
2. Obi-Wan would become more powerful than Vader could possibly imagine
Unless the most powerful thing Darth Vader can imagine is a partially-transparent advice-giving ghost, Obi-Wan's kinda full of bullshit here. He actually becomes pretty much entirely powerless - he gives Luke some tips (as a voice in his head) and chats with Yoda a little. That's not very powerful at all, especially considering he's standing on a space station that literally blows up planets when he makes this claim. That alone is infinitely more powerful than what Obi-Wan becomes.
You know what Obi-Wan should have done with his ghost-powers? Mess with Vader CONSTANTLY. He's now a ghost who can show up anywhere in the galaxy at will. Just annoy Vader for the rest of his life. Follow him around and scream at him, keep him up at night by yodeling, tell everyone in the Empire embarrassing secrets about Anakin's past ("Ol' Darth there's childhood nickname was 'Annie.' Haha, no, I'm totes serious, you guys!"). You're a GHOST. Why not actually make use of that ability?
Then again, maybe he thought hanging around in a newbie pilot's head and chatting with a swamp Muppet was a better use of his ultimate power.
3. The Dark Side isn't stronger than the Light Side of the Force
YES, YES IT IS. Are you kidding, Yoda? Hell, you were even BEATEN in a big Dark vs. Light battle, which is why you're living in a swamp by yourself tricking farmboys into giving you piggyback rides.
The Dark Side has FORCE-LIGHTNING. What does the Light Side have? Turning yourself into a helpless ghost? Lifting rocks? And if you wanna go extended universe on this (which is never a good idea), THE DARK SIDE CAN CAUSE FORCE-WORMHOLES THAT CAN RIP WORLDS APART. Meanwhile, the Light side has such kickass powers as "animal friendship" - which looks like this:
Plus, the Light Side is full of lame-o Jedis who aren't allowed to have girlfriends or show emotions ever. The point is, the Dark Side rules.