Which one should i buy? a gps? or a fish finder?  I don't want to spend over 200 dollars.  I was looking at a Garmin 250c (fishfinder) for 200 dollars.  What do you guys think?

I'm not really familiar with Garmin... I'm also looking for a FF but I'm convinced that Lowrance is the way to go. I know a guy with a X-135 and its pretty impressive.

yea the lowrance x 135 is nice but its WAY out of my price range.

If you only have money for one of them and can't afford a combo, buy a fish finder.  You will be able to find structure and bait balls with it and when you find a good spot you can make note of where it is in relation to particular landmarks on shore.  (You can also mark it on a map.)

Is there a real big difference b/w cheaper fishfinders and the really expensive ones?  What do I really need? water temp..depth...structure..

There's a huge difference.

The higher dollar sonar units are more sensitive, more effective, easier to read.

Far as fishfinder vs GPS, IMHO your comparing apples to oranges.

If you don't have a fishfinder, you'll need that first. With it you can see what's below the water. A GPS would ne next, it'll get you back to what you find below the water.

BTW, the Garmin 250C is a decent unit, I have one. If you keep looking you may find one a little cheaper than $200. I paid $125 for mine, used of course. Combine that with a handheld GPS and your set.

If you will look closely the lowerance and eagle units are the same except for price.The eagles dont pay sponsorships like the lowerance does.The thing I look for in a depth finder is a high pixel count. The eagle fishmark 480 has 480X480 pixels for $199.00. The fishmark 320 has 320X320 pixels. A friend of mine had a garmin 90. It had 128X64 pixels. Structure was almost unreconizable because the screen had such large pixels. I have two 320s on my bass boat and one 320 on my bass hunter. I love them.