I have noticed all kinds of setups. 6:1, 7:1 and 8:1
What are the advantages to each one? Other than the lower ones can be cheaper? The higher the purchase ratio the easier to pull in! DUH! But in a race situation, is it posible that a mainsheet can give you an edge? 
What are the pros and cons of each setup if any? 
With a higher purchase you will have more sheet on the deck. It will take longer to sheet in changing from downwind to upwind. The extra line is also easier to get tangled up or slip through the laces and out the stern. I've always thought that using the smallest purchase that you can and still properly sheet in the main in any wind condition works best. If you can fully sheet in your main in a 20 know wind with a 7:1 purchase, you don't need the extra line of the 8:1 purchase. If it's too hard than you should add the extra purchase. It also depends on what kind of sailing your doing. Pleasure cruisers will just want it easier. Long distance racers might want it easier as well to be less fatiguing.
Lance has it right...for me, I need to have enough purchase that I can handhold the mainsheet (NO CLEATING) in a 20 knot blow. My 6.0NA had 7:1 which, in my opinion, was not enough. My F18 has 8:1 which is very comfortable and easy to operate in the heavy air. More purchase makes it easier to cleat and uncleat while tacking but as you both said, the drawback is extra line.
And both are great answers to the question. One of the problems that made folks stick with less purchase was the drag when you wanted to let out your sheeet quickly.
Until recently all we had were ratchet blocks that turned in only one direction. To let the sheet back out, it had to go across a block that was not turning.., thus a lot of friction and sometimes the sheet simply did not go out.
That inability to let the sheet out quickly can cause capsizes in puffy winds; it will cause blown tacks (for a perfect cat roll tack you need the mainsheet in tight to initiate the turn and just as the sail goes head to wind, you need to release and ease some sheet -- that easing allows the leach of the main to ease off and let the boat complete its turn
Thanks to Harken, we now have Ratchamatic Mainsheet Blocks that really do the job -- they ratchet when you pull and hold the sheet, but when you release the pressure, they become free-wheeling. That allows the sheet to ease back through the blocks easily.
And, due to the Ratchamatic, you now can use higher purchases.
Still another good thing that has happened is the new line available for sheets. Modern line has all of its strength in the core. And now a lot of the line have SUV protected cores. So, you can strip the cover off the part that runs through the blocks and leave the cover on the part that you hold.
That really allows the sheet to feed back through.
You can take a look at the different sheeting systems on our store at:
Hope this helps. http:/