Quick Rear Derailleur Upgrade Question
 

So I've upgraded pretty much every component on my bike except for the rear derailleur. The bike started out as a Cannondale Quick 6 with a 7 speed Suntour cassette and a Shimano Altus rear derailleur.

I put on an 11-28 Shimano 8 speed cassette (HG70-I) and want to upgrade the rear derailleur. I've done a lot of searching on here and it looks like I need a long-cage mountain derailleur. Rapid rise is fine, too, but means that shifting is backwards from what I understand. If it helps any, I am using Shimano 8 speed bar end shifters (which apparently aren't DuraAce compatible).

I've narrowed it down to either:
1) Shimano LX RD-M580 (long cage, SGS, rapid-rise) for 44 bucks shipped
2) Shimano M771 XT (long cage, top-normal) for 57 bucks shipped

My question is whether or not these will work and which one I should go with. If you guys have any other suggestions on rear derailleurs that would be awesome. The bike came with bargain components and I've been upgrading them progressively with nicer components, but nothing too expensive (because the bike wasn't that much to begin with).

Thanks in advance :)

Any dérailleur will work for you.

Cool, I'm really leaning towards that M771 since it's a really good deal.