Kegs and stir plate

First question,
whats the difference between soda kegs and cornelius kegs? ball lock and pin lock?

and...

whats the best stir plate that wont break....northern brewers (too expensive)???

also which keg is better?

Soda kegs and cornie kegs are the same thing - well, more accurately, Cornelius is a very common manufacturer of soda kegs. Basically, the terms are interchangeable.

Ball lock and pin lock kegs refer primarily to how the quick disconnects attach to the kegs. The pin lock kegs, I think, tend to be a little cheaper - not sure if it's because they're more plentiful or they're less in demand. Also, many pin lock kegs lack the pressure bleed-off valve that can be found on ball-lock kegs. This valve makes it easier to purge gas from a keg.

Which is better is definitely up for debate, but in general ball-lock are the more popular option.

For a stir plate, I just got on from stirstarters, and it certainly seems sturdy enough. It also seems to be pretty highly regarded on HBT.

Pin-lock kegs are in greater supply because they have a larger diameter than ball lock kegs I believe they are 8.5" and 8" respectively. This function of diameter comes into play when you are trying to cram as many kegs as you can into a keezer/kegerator. Ball locks are generally preferred because they take up less floor space (but are taller which is compensated for with a collar).

The least expensive stir plate is going to be one you make from a trip to radio shack and spare computer parts (check the DIY thread there is lots of info).

Get whichever keg you can get cheep and local. For stirplates look on ebay, I would even conciser one with heat built in.