It would be best to buy the one recommended for your machine, which for the Macbook is the 60 watt and the 85 watt for the Macbook Pro. (At least the older ones. I have not looked that info up on the new unibody models.)
Don't personally find a problem. I have been using the same power cord now for 28 months on my MBP. It is used while I sit in my recliner, with the cord running to the wall on the opposite side of where it plugs into the machine. It has no visible signs of wear and tear and it has been used in this way for a minimum of 4+ hours a day for that entire time.
I do take reasonable care of it and don't allow it to become kinked where it needs to bend heading back to the wall outlet. (The fact that I'm around 60 and don't treat my stuff the way a lot of kids do (even when they believe they've maintained utmost care) probably has some impact on this also.)
If you're close to an Apple store, you might try taking it in to the store. Apple might surprise you. Even outside of warranty, many people have gotten free replacements for these.
(And, yes, I do realize that there was a real issue with some of the magsafe power cords. We won't go there here.)
