Belt tightening vs replacement?

Which is the better deal? I got quoted $200 for a belt tightening, and after some research it looks like it's the same cost as just replacing the belt. I figure why not get a new belt while I'm at it. But I'm not a car expert so i don't know if I'm right about the cost. And maybe there's some techincal issues that a new belt may cause that I'm unaware of. I don't know

So, tl;dr: Should I get my belt replaced or just tightened? If not why?

Not sure if this matters but I have a 2006 Honda Civic with 104,000 miles on it.

Your 2006 Civic does not have a timing belt. All the engines the 2006 Civic came with in the US use timing chains.

What is in question is your v-groove serpentine belt. I have no idea why a professional would suggest tightening an 8 year old v-groove belt. It is worn out and tightening will, at best, help for a few weeks. And the only way he can do it is to replace the automatic tensioner (which should always be replaced when the serpentine belt is replaced). What your car needs is a new serpentine belt and a new tensioner. Check out the excellent video in the source for more on that. At 8 years old I'm sure the old belt will show the subtle but unmistakable signs of being worn out. The grooves won't have the sharp V at the bottom but will instead have a wide flat valley. The ribs grip on the sides so no amount of tightening will ever make up for the wear. I have used tightening manually tensioned serpentines to get a car to where I can change the belt but I have never had it help more than 100 miles.

Something is not right here, First off if a timing belt, no reputable shop will do it for $200 bucks, and they sure as hell wouldn't offer to tighten any belt, You replace them, Period. Second, by 2006 Honda switched over to timing chains for most of there engines, so there's no timing belt in most cases, If this shop is offering to tighten your accessory belt. I'd really like to know how they plan to do that, as it's done automatically by a tensioner, and there's no adjustment, If you can't find a shop that will replace the accessory belt for $75 bucks including the price of the belt your not looking hard enough, Now If they are talking about replacing the tensioner $200 is about right including the price of the tensioner, Why you wouldn't spent the extra $30 bucks on a new belt is doesn't make a lot of sense, as to tightening the belt that sounds fishy, I'd go get another estimate, you may be about to be ripped off

At 104K it is time, To replace Both the Timing and Accessory belts, and the accessory belt tensioner should be replaced also at the same time.

You didn't mention which belt.
If it's the Timing Belt and it is old, replace it.

Which belt? You are getting ripped off it it is the external belt. It sounds like you don't have a timing belt, so I don't know what you would be fixing for $200.