Mountain Bike Derailleurs

Whats the difference between these rear derailleurs?
I have an old 6 speed Schwinn that the rear derailleur broke off.
I’m looking for a replacement.
But I don’t under stand this with and without hanger.
It’s a Shimano Tourney.
RD-TX51:
http://www.amazon.com/Shimano-Tourney-RD-TX51-Derailleur-without/dp/B000F5EFXS/ref=sr_1_1?ie=UTF8&s=sporting-goods&qid=1279149783&sr=8-1
http://www.amazon.com/Shimano-RD-TX51-Tourney-Derailleur-Hanger/dp/B000FFQ4LE/ref=sr_1_4?ie=UTF8&s=sporting-goods&qid=1279149783&sr=8-4
I also saw a RD-TX31 which is also for 6/7 speeds.
Which is better?
This kind of equipment like mountain bikes isn’t my expertise.
and which would you recommend?
oh any my Schwinn is a 3 ring crank set
It depends on your frame, some frames come with a hanger and some don’t. A hanger is a hole at the bottom of the frame near the hole where the rear wheel bolts up that allows the derailleur to bolt to the frame.
If your frame doesn’t habe a hanger, then you have to have a derailleur that bolts to the wheel stud and a screw goes through a hole in the frame to secure it.
this is a picture of a hanger, the hanger is the hole at the very bottom.
http://www.headshok.com/bikes/03/images/f400_tech_03.jpg
If your bike doesn’t have one, order the derailleur in the first link. I dunno why it says without hanger for the first link, because that derailleur is for bikes with a hanger according to the picture. If your bike doesn’t have a hanger, get the second one, and again, its actually for bikes without a hanger according to the picture.
Mechanical Parts & Concepts of Mountain Bikes : How To Install A Mountain Bike Derailleur

